The aims of the investigation were: 1) to determine if there are defects in interleukin 2 (IL 2) regulation either on phytohemagluttinin (PHA)-activated or non PHA-activated peripheral blood mononuclear cells (PBMC) in cancer patients, in order to ascertain the role of IL 2 in this disease, and 2) to carry out preliminary experiments for a direct quantitative evaluation of endogenous IL 2 production by PBMC cultures of cancer patients. An assessment of lymphocytes subsets was also performed with monoclonal antibodies in a selected group of patients. A total of 159 patients entered the study. Cancer sites were: larynx, 49; breast, 42; lung (NSC), 25; colorectal, 18; and gynecologic, 25. In the former 3 cancer sites, staging showed localized or only locally advanced disease and in the later 2 sites it showed disseminated disease. Our results provided evidence that the cancer patients exhibit a T cell functional immunodepression, which progresses during tumor growth, so that the localized disease shows a low-grade defect and advanced disease shows a high-grade defect. Our data also clearly suggested that the factor involved with a primary role in this functional immune impairment is the IL 2 deficiency. In our study we have not found a substantial difference of activity between recombinant and nonrecombinant IL 2, although the comparison of the relative activities of the two types of IL 2 is not easy to make, since they are expressed in different ways; however the recombinant one appeared to be slightly more active, probably for the higher purity. Our data also seem to support the perspective of the in vivo therapeutic administration of IL 2 in cancer patients.