Glucose 1,6-biphosphate (G1,6P2) was measured in human, pig, cow, rabbit, rat and sheep red blood cells. Mean values are variable among the species and range from 33 to 122 nmol/ml RBC for pig and rabbit erythrocytes, respectively. The activities of G1,6P2 synthase, phosphoglucomutase (PGM) and phosphoribomutase (PRM) have also been assayed in red cell haemolysates of the same species. The correlations between the biphosphate content and the occurrence of the three enzymatic activities have been studied in order to gain an insight into the regulation of the G1,6P2 turnover in mammalian erythrocytes.