The cross-reactivity of human phosphoglucomutase isozymes (PGM1, PGM2, PGM3 and PGM4) has been investigated using anti-rabbit muscle PGM polyclonal antibodies. Significant differences were revealed: an IgG fraction of the antiserum reacted with the primary and secondary PGM1 isozymes of all the common phenotypes. However, there was no reaction with the PGM2 or PGM3 isozymes; thus these latter isozymes share no major antigenic determinants with human or rabbit PGM1 and are therefore structurally distinct. In contrast, the PGM isozymes of human milk attributed to a fourth locus, PGM4, showed similar cross-reactivity as PGM1 suggesting close structural similarity. The IgG was also employed as a reagent to remove PGM1 from haemolysates so as to allow the unambiguous assessment of the PGM2 isozyme patterns by isoelectric focusing. However, no proven genetic variation was encountered in a sample of 32 individuals.