The distribution among peripheral blood mononuclear cells of latex-ingesting cells, T cells, and B cells was determined with samples from 38 normal donors and 25 patients with malignant melanoma. The mean percentage of latex-ingesting cells, as well as B and T cells, was significantly reduced in the 25 patients with malignant melanoma, stages I and II, compared to the controls. Several explanations for these unexpected findings were considered; possibly the presence of occult neoptasm was responsible for the observed changes in cell distribution.