Some serological (serum immunoglobulins, circulating immune complexes, quantitative assessment of hemolytic complement activity) and cellular (total number of circulating lymphocytes as well their phenotypic and functional characterization) parameters have been analyzed in 42 patients with lung cancer of three hystologic types and in 19 healthy controls to define whether different histologic neoplasms have an impact on the host immune system. As a group, cancer patients showed 1) the presence of circulating immune complexes, in a significant percentage, within all the three groups studied; and 2) a significant reduction of lymphocytes forming E rosette. With regard to cellular immune parameters we noted that 1) T lymphocytes isolated from the peripheral blood lymphocytes (PBL) of patients with lung cancer showed a decreased capacity to express HLA-class II antigens upon phytohemogglutinin (PHA) activation in vitro; 2) PBL from lung cancer, in particular the adenocarcinoma group, failed to proliferate upon stimulation by policlonal mitogens; and 3) the autologous mixed lymphocyte reaction (MLR) of non-T/T-type was impaired in all three groups, whereas the autologous MLR of T/T-type was impaired only in the patients with squamous cell carcinoma and in those with adenocarcinoma, probably as a result of a reduced stimulatory capacity.