By x-ray crystallography of the 116 epimer, toxins C3 and C4 are shown to be 2lsuIfo-N-Ihydroxysaxitoxin-llo- and Ilf%-~ydroxysulfate, confirming the position and identity of the 3 substituents which, with the parent compound, form the array of 12 saxitoxins found in Protogonyaulax.In the analysis of toxins from cultured Protogonyaulax of the northeast Pacific (2) the expected compounds 1, & 5 3 5 and 11 were generally found to accompany somewhat larger amounts of the 2Isulfo derivatives !& s g, and J& Although composition varied greatly among isolates, the above substances were al1 eventually found in relatively high concentrations. Compounds g (C3) and 12 (C41, the corresponding derivatives of z(gonyautoxin 1, GTX 1) and fi(gonyautoxin 4, GTX 4) were at first notable for their absence but were finally detected in the mother liquors from crystallizations of 4 and 6 (3), and as trace constituents in analyses of Protogonyaulax clone PI07 (4).The behavior of 10 and 12 (5,7) is largely analogous to that of 4 and 5 (2,3). Compounds g and 12 - epimerize (TLC, NMR), the conversion of 12 to 10 predominating. Hydrolysis (O.lM HCl, 100°C, 5 minutes; 6) - converts 10 to 2 and 12 to 5 (TLC). Preliminary assays indicate that the mouse intraperitoneal potencies of - crude 10 and 12 increase by factors of 40x and 7x with these conversions.