The cytochrome P450 mono-oxygenases are a group of enzymes that metabolize a variety of exogenous and endogenous compounds, some of which are potentially toxic. Individual variations in the metabolism of potential toxins could influence susceptibility to disorders having genetic and environmental components, such as schizophrenia. The frequency of two common mutant alleles of the gene for the cytochrome P450 enzyme debrisoquine-4-hydroxylase (CYP2D6) was determined in 264 Caucasian schizophrenic patients and 217 controls, using the polymerase chain reaction and restriction enzyme digestions. The patient and control samples showed no significant deviation from Hardy-Weinberg equilibrium and the frequency of each mutant allele (CYP2D6A and CYP2D6B) did not differ between patients and controls.