Eight rare variants of human PGI have been discovered during a survey of 3,300 blood samples from various parts of the world. Seven of the 8 variants had altered electrophoretic mobilities. One of these variants was only discovered by the use of a second buffer system. Two of the 8 variants had half the normal level of activity in red cells and white cells. Apart from the partial deficiency of 2 of the variants, the 8 variant phenotypes were not found to differ significantly in any of the following properties: Heat and urea stability, Km (F6P) and Ki (6PG), activity and antibody inhibition.