Three post-infective cold agglutinins, two of which were associated with human infection with Mycoplasma pneumoniae were purified and compared with cold agglutinins isolated from three patients with chronic autoimmune haemolytic anaemia (CHA) as regards their immunoelectrophoretic patterns, their light chain types and the appearances of their light chain zones (after reduction and alkylation) in starch gel electrophoresis. One of the M. pneumoniae-induced cold agglutinins was indistinguishable from CHA cold agglutinins for it was monotypic (type-K), had a restricted mobility in immunoelectrophoresis and a narrow, well-demarcated light chain zone in urea formic acid starch gel electrophoresis. The other two post-infective cold agglutinins though bitypic with respect to light chains showed a considerable degree of homogeneity in immunoelectrophoresis and in starch gel electrophoresis they had light chain bands which were sharper than the light chain zone of normal human IgG.