The distribution of platelet monoamine oxidase (MAO) activity is examined in a large cohort of 18-year-olds from a college setting. A mixture of three distributions is needed to describe the data, even when a power transformation is used to remove skewness in the distribution. This is compatible with MAO activity being controlled by a single major locus with a gene frequency of 0.02 for high-MAO activity. Accordingly, it is unlikely that such a locus could serve as a genetic marker for a disorder which is associated with low activity. However, this finding does not rule out the possibility that MAO activity is an associated risk factor in disease.