Peripheral blood lymphocytes and their in vitro reactivity have been recorded prior to treatment in 18 patients with Hodgkin's disease, 11 with lymphosarcoma, 13 with reticulosarcoma, 20 with various solid tumors and 37 normal control persons. The mean total numbers of lymphocytes, those of T lymphocytes, and the mean reactivities to PHA and ConA were reduced in all groups except lymphosarcoma, although with varying degrees of statistical significance. The percentages of T and B lymphocytes appeared to be normal in all groups, but the ranges of values were somewhat greater than among the normal controls. The mean total numbers of B lymphocytes were normal in all patient groups. All reductions seemed to be more pronounced in patients with disseminated than in those with localized disease, but none of these differences was statistically significant. All patient groups appeared to have reduced reactivity in MLC, while the ability to stimulate control lymphocytes was nearly normal. The results fail to indicate any in vitro immunological abberation specific to Hodgkin's disease. It seems that human malignant, neoplastic diseases are associated with a relatively selective reduction of the total numbers and reactivity of blood T lymphocytes. Various explanations of the reactivity impairment are proposed. The pathogenesis of the reduction of the total number of blood T lymphocytes remains obscure.