In a holoendemic malaria region of Nigeria children of both sexes aged from 9 months to 6 years with an acute febrile illness were examined for malaria. In 461 children with malaria, predominantly P. falciparum, and in 241 children without malaria, haemoglobin levels, haemoglobin types, red cell glucose-6-phosphate dehydrogenase variants and parasite densities were recorded. The G6PD status was determined by a combination of spectrophotometric enzyme quantitation, electrophoresis and the cytochemical methaemoglobin elution technique. Malaria morbidity and parasitaemia decreased with increasing age. Frequencies of haemoglobin types and G6PD variants were not significantly different in the malaria and the non-malaria series. Haemoglobin values were significantly lower in children with malaria to about the same extent in HbAA and HbAS subjects, but no close correlation existed between haemoglobin level and parasite density. Details of the G6PD classification and the effect of malaria on enzyme activity are discussed.