The authors report on their research which sought to find an immunological marker for suicidality. The frequency of NK-cells & their activity in the presence & absence of serotonin was examined in 19 subjects who had attempted suicide & 19 matched controls. Results suggest the following peripheral immunological markers in the suicide attempt subjects: decreased NK-call frequency, diminshed NK-cell activity & a correlation between low sensitivity of NK-cell activity to serotonin stimulation.