After the reaction of reduced BSAt with BPA at pH 10·6 and subsequent hydrolysis of the protein with 6N HCl, only 40% of the lysine present in the unmodified protein can be accounted for on chromatographic analysis of the hydrolysate; a similar loss is observed of the SAPC into which the original cystine, which has totally disappeared, is presumably initially all converted. In place of these amino acids, a "neutral" ninhydrin-positive peak is observed which accounts approximately quantitatively for the missing lysine. The reaction at pH 8· 6 with IP A leads to a quantitative recovery of SAPC with no loss of lysine. In contrast, reaction for the same times at both pH values with BEA leads to quantitative recovery of all amino acids including SAEC. Although the reaction ofIP A with thiols at pH 8·6 is very slow compared with that of other alkylating agents, the relative absence of side reactions may be compared with the state of affairs when methyl iodide is used for an equivalent period in the same relative quantities, when there is extensive alteration of lysine, histidine, methionine, and S-methylcysteine.