Multiple in vitro immune parameters were investigated in thirty-four untreated patients with invasive carcinoma of the cervix and in twenty-five controls. The parameters measured were percentages and absolute counts of T and B cells, percentage of T cell subsets, lymphocyte response to phytohemagglutinin (PHA) and concanavalin A (Con A), natural killer (NK) activity, antibody-dependent cellular cytotoxicity (ADCC), and interleukin 2 (IL-2) activity. Patients with invasive cervical carcinoma, as compared with controls, showed a decrease in the percentage and count of T cells, a decrease in the percentage of helper-inducer (CD4+) T cells, decreased CD4+/CD8+ ratio, depressed lymphocyte response to PHA and Con A, and depressed NK and ADCC activities. There were no significant differences in these immune parameters between early and advanced tumor stages. The levels of total lymphocytes, monocytes, suppressor-effector (CD8+) T cells, and B cells were similar to those of the controls. IL-2 productivity in patients was lower than that in controls. In patients with invasive cervical carcinoma, a decrease in the percentage of CD4+ cells was associated with depressed PHA response and decreased IL-2 productivity was correlated with the reduced percentage of CD4+ cells and decreased NK activity. This study shows a significant defect in an important immune surveillance mechanism in patients with invasive carcinoma of the cervix and suggests that impaired IL-2 activity production may be related to quantitative and qualitative alterations in lymphocyte subpopulations which play a major role in immune surveillance against cervical cancer.