The ability to carry out the initial reaction of the pentose phosphate pathway was investigated with extracts of mouse erythrocytes infected with Plasmodium chabaudi, purified merozoites of P. knowlesi, and schizonts of P. falciparum grown in vitro in human erythrocytes. Glucose-6-phosphate dehydrogenase activity (G-6pd) was detected in extracts of all the cells after electrophoresis on polyacrylamide gels. Separate host cell and parasite glucose-6-phosphate dehydrogenase activities were demonstrated with extracts of P. knowlesi and P. falciparum but not with P. chabaudi.