The red cell PGM activities have been determined in 95 individuals with PGM11 phenotype, in 86 with PGM12–1 and in 18 with PGM12 phenotype. The average activities of these three groups were similar, a finding which strongly suggests that the measurable activities associated with the two common PGMX alleles are identical. The PGM phenotype has been determined in 389 samples of blood from Rome. All of them were PGM21. One variant PGM1 pattern has been found corresponding to the PGM16–1 phenotype first described by Hopkinson and Harris [1966]. The PGMt gene frequencies were: PGM11 = 0.712; PGM21 = 0.287; PGM61 = 0.001.