In a controlled study the activity of glucose-6-phosphate dehydrogenase (G-6-PD) in red and white blood cells, γ-glutamyl transpeptidase (γ-GT) and lysozyme in serum and white blood cells was studied in 22 drug-free schizophrenic patients and 17 healthy volunteers. The activities of the above enzymes were found to be reduced in the white cells of schizophrenics compared with controls. The differences in activity of G-6-PD in red cells and of γ-GT and lysozyme in serum between the two groups were not revealed as significant. The observed low enzyme activities might provide a further basis for interpreting the reported functional deficiency in neutrophils of schizophrenics. Possible mechanisms in relation to biological abnormalities in schizophrenia are discuss