Starch gel electrophoretic patterns of 26 enzymes (corresponding to 36 gene loci) were examined in hemolysates of erythrocytes from 11 first-trimester and mid-trimester human fetuses (65-138 gestation days). The zymograms of 16 enzymes were identical in fetal and control adult red cells. Six enzymes (enolase, guanylate kinase, lactate dehydrogenase, nucleoside phosphorylase, phosphofructokinase, hexokinase) showed differences in the staining intensity of certain isozyme zones as compared with the controls. Also, the fetal red cell zymograms, in contrast to those of adults, contained the mitochondrial forms of isocitric dehydrogenase and glutamic oxaloacetic transaminase as well as more definite zones of phosphoglucomutase-3. Finally, some of the isozymes of uridine diphosphate kinase in the fetal cells had slightly retarded mobility. These observed differences between fetal and adult red cells could reflect the expression of a different program of protein synthesis in red cells of the fetuses or the epigenetic modifications of isozymes in immature red cells.