Partial reduction and alkylation of a Waldenström IgM followed by reaction with citraconic anhydride resulted in modification of 92-95% of the free amino groups. The citraconylated μ and κ chains were obtained in pure form by chromatography on Sephadex G-200 in a nondenaturing solvent. The molecular weight determined from the sedimentation and diffusion coefficients for the citraconyl μ chain was 83 000 and that for the citraconyl κ chain was 23 000. Substraction of the weight contributed by the citraconyl groups gives estimated residual molecular weight for μ chain of 77 000 and for κ chain of 22 000. Deblocking of citraconyl μ and κ chains was achieved by incubation at pH 4 for 3 h. The deblocked chains showed a multiple banding pattern in acrylamide disc gel electrophoresis. Peptide maps of tryptic digests of deblocked citraconyl μ, maleyl μ and μ chains obtained by conventional methods showed considerable susceptibility to tryptic digestion. © 1972.