In the course of study of the acid hydrolyzate of a polysaccharide isolated from Bacillus subtilis 1, a substance moving on paper chromatography faster than known hexosamines was observed with Rglucose 1.4, in butanol--ethanol-water, 4:1:1 (Solvent I) and Rglucosamine 1.38, in n-propanol-1% ammonium hydroxide, 7:3 (Solvent II). The spot reduced AgNO3 and was positive to the ninhydrin and Elson-Morgan reagents. In order to prepare a large amount of this substance, 7.8 g of unpurified polysaccharide was hydrolyzed with 1 N H2SO4 for 6 h at 100 °. After neutralization with BaCO3, the hydrolyzate was chromatographed on a Dowex-50 column according to GARDELL 2. The substance could thus be separated from glucosamine and galactosamine, moving much slower. After isolation of the corresponding peak and further purification by paper chromatography, 112 mg of crude material was obtained. Crystallized from a mixture of water, methanol and acetone, it yielded 71 µg of colorless prismatic needles. This substance (Compound A) was homogenous as determined by paper chromatography in five different solvent mixtures. It melted at 216--219 °, with decomp, starting at 200°; [α]D²³ +115 ° (after 5 min) to +94 ° (23 h), in water. (Found: C, 40.16, 39.99; H, 7.05, 7.03; N, 11.60; Cl, 14.62; CH3, 10.5. Calc. for C8H17O4N2Cl: C, 39.90; H, 7.12; N, 11.64; Cl, 14.73; CH3, 12.5). Assayed for reducing sugars 3, hexosamine 4, and free amino groups by ninhydrin 4 and by alkaline distillation 5, the molar values were between 0.8 and 0.95 using glucosamine hydrochloride as a standard. The spectrum of the Elson-Morgan color was the same as that of glucosamine. Periodate oxidation liberated 0.4 mole acetaldehyde 6, no formaldehyde 7, and 1.7 moles formic acid 8. Degradation with ninhydrin gave a reducing substance with Rglucose 5.8 (Solvent I). On paper electrophoresis in borate buffer at pH 10, it moved with a Mglucose 0.7, while Compound A did not exhibit any movement at all. Action of 4 N HCl on Compound A for 2 h at 100 ° gave a product with Rglucosamine 1.09 (Solvent II), reducing AgNO3 and giving a positive reaction with ninhydrin and the Elson-Morgan reagent. By N-acetylation of Compound A, a product (Compound B) was obtained, crystallizing from a mixture of ethanol and ethyl acetate in fine needles or plates, m.p. 262-264 ° (decomp.); [α]D²³ +67 °, in water--ethanol, 1:1; Rglucose 4.5; Mglucose 0. (Found: C, 49.36; H, 7.90. Calc. for C10H18O5N2: C, 48.77; H, 7.37). It gave no color in the Morgan-Elson reaction. The infrared-absorption spectrum of Compound A showed strong absorptions at 1660 cm⁻¹ (amide I), at 1530 cm⁻¹ (amide II) and at 3365 cm⁻¹ (secondary amide), as well as at 3300 cm⁻² and 3550 cm⁻¹ (primary amine), but lacked an absorption at 1725-1749 cm⁻¹, characteristic of the O-acetyl carbonyl group. The infrared-absorption spectrum of Compound B showed strong absorptions at 1640 cm⁻¹ (amide I) and at 1540 cm⁻¹ (amide II), but had only one absorption in the 3000-3500 cm⁻¹ region (3355 cm⁻², secondary amide). Based on the preceding observations, it is possible to propose for Compound A the formula of a 4-acetamido-2-amino-2,4,6-trideoxyhexose and for Compound B the formula of a 2,4-diacetamido-2,4,6-trideoxyhexose. Thus, it appears to be the first isolation of a diaminohexose from natural sources. Evidence for the production of heparin by mast cells has been obtained by a variety of indirect techniques. These include the demonstration of (1) metachromasia in cells stained with toluidine blue 2; (2) a parallelism between the concentration of mast cells and apparent heparin content of tissues 1; (3) S uptake by mast cells of rats * and by heparin of mast-cell tumors both in vivo 8 and in vitro *,9; and (4) the isolation of heparin from mast-cell tumors 8. Although synthesis of heparin has been demonstrated in mastocytomas 8-5, there is no direct chemical evidence for the presence of heparin in the normal mast cell. In view of the hypothesis that hyaluronic acid is formed by mast cells 9, it seemed desirable to obtain direct chemical evidence regarding the nature of the acid mucopolysaccharides present in mast cells. Mast cells were obtained from the peritoneal fluid of 500 normal, male Sprague-Dawley rats by the method of PADAWAR AND GORDON 10, using a 0.9 % NaCl solution containing 0.15 % disodium ethylenediaminetetraacetate for flushing the peritoneal cavity. Approximately 1.1×10⁶ mast cells were isolated from the peritoneal fluid of each rat. Differential counts indicated that these cells comprised 90 to 98 % of the total cell population of each preparation.