Mice immunized with the recombinant antigen 11.1 beta-galactosidase, consisting of 22 repeats of the nine-amino acid unit from Plasmodium falciparum antigen 11.1, produced antibodies reacting with human serum albumin. A positive reaction was observed in dot-blot assays, in enzyme-linked immunosorbent assay and on immunoblots of sodium dodecyl sulfate polyacrylamide gels as well as two-dimensional gels. Binding was specific for human albumin, as no reaction could be detected on bovine serum albumin, hen egg ovalbumin, rat serum albumin or another abundant human serum protein, the alpha 2-macroglobulin. In addition, rabbit antibodies raised to human serum albumin reacted with keyhole lympet hemocyanin coupled to synthetic dimers of the nine-amino acid repeats of the P. falciparum 11.1 antigen. These data indicate antigenic relationship between the 11.1 antigen and human albumin. The proteins have a short sequence of homology in a region where human serum albumin differs from the albumins of other species.