The relationship between suicides and suicide attempts and two biological measures, platelet monoamine oxidase levels (MAO) and average evoked response (AER) augmenting, was examined in 79 off-medication psychiatric patients and in 68 college student volunteers chosen from the upper and lower deciles of MAO activity levels. In the patient sample, male individuals with low MAO and AER augmenting, a pattern previously associated with bipolar affective disorders, showed a significantly increased incidence of suicide attempts in comparison with either non-augmenting low MAO or high MAO patients. Within the normal volunteer group, all male low MAO probands with a family history of suicide or suicide attempts were AER augmenters themselves. Four completed suicides were found among relatives of low MAO probands where as no high MAO proband had a relative who committed suicide. These findings suggest that the combination of low platelet MAO activity and AER augmenting may be associated with a possible genetic vulnerability to psychiatric disorders.