The isolation of erythromycin, a clinically useful antibiotic, from Streptomyces erythreus has been reported. Paper Chromatographic methods, developed primarily to aid in isolation and fractionation of erythromycin, gave evidence that more than one antibiotic substance can be produced by certain strains of Streptomyces erythreus. Details of the isolation and partial characterization of a second antibiotic substance, erythromycin B, are presented in this paper. High concentrations of thiamine in alkaline solution catalyze several reactions of carbonyl compounds known to occur in biological systems, among them the formation of acetoin and acetate from acetaldehyde and diacetyl. Acetaldehyde-C¹⁴ has been employed to demonstrate the mechanism of this reaction.