Cell mediated immunocompetence was measured serially in 35 patients with malignant melanoma in order to determine the effect of extent of disease and prognosis as well as the influence of BCG immunotherapy on immune reactivity. Compared with normal adult controls, statistically significant lymphopenia occurred only in patients with widespread disease. Seventeen of 21 patients with negative pre-therapy PPD skin test converted to skin test positivity. PHA blastogenesis was depressed only in patients in the pre-terminal stages of their disease using optimal mitogen concentrations for stimulation. Threshold concentrations of this mitogen more clearly demonstrated a depressed responsiveness which correlated in severity with extent of disease. PPD induced blastogenesis was normal or increased in the majority of patients; however, the degree of stimulation by PPD was less in the BCG induced convertors than in those patients who were skin test positive before BCG treatment. Comparison of the pre- and post BCG assessments reveals no significant differences except in relation to PPD conversion. We conclude that using threshold concentrations of PHA, impaired responses are regularly associated with disease beyond the regional lymph nodes. Routine assessment of lymphocyte function by these parameters did not provide information that was not available from clinical evaluation.