Human phosphoglucomutase activity has been determined in red blood cells obtained from 348 unrelated subjects. The mean activities attributed to the four common PGM1 alleles, expressed as micromoles of G6P produced per gram of Hb per hour were 53 for PGMa31, 60 for PGMa11, 61 for PGMa41 and 72 for PGMa21. The relative amount of variation associated with the electrophoretic polymorphism was estimated as 24%.