We prospectively evaluated responses to recall antigen in ten cancer patients undergoing immunotherapy and correlated these responses with in vitro proliferation data. Before therapy, eight of ten patients responded normally to at least two of seven antigens of a multitest system (greater than or equal to 2 mm induration at 48 hours), with a mean induration score of 17.9 +/- 4.4 mm and 2.7 +/- 0.5 positive responses per patient. This decreased to 5.9 +/- 2.7 mm (P = .01) and 1.2 +/- 0.5 responses (P = .03) after a week of interleukin-2 (IL-2) therapy, and further to 0.7 +/- 0.7 mm and 0.1 +/- 0.1 positive responses during a second week of therapy consisting of IL-2 plus activated autologous lymphocytes (P less than .01). The in vitro proliferation indices for lymphocytes obtained before skin test application were significantly less after IL-2 compared with pretreatment for concanavalin A ([con-A] Miles Laboratory, Elkhart, IN) stimulation (3.3 +/- 0.7 to 1.3 +/- 0.1; P = .03) and in mixed lymphocyte culture (MLC) (41.5 +/- 8.5 to 16.8 +/- 3.8; P = .02), and during the second week of therapy for in vitro IL-2 stimulation (83.3 +/- 16.8 to 42.9 +/- 12.0; P less than .01). When skin responses were directly compared with in vitro proliferation data, a significant correlation was observed for tetanus (r = .75; P less than .01), streptococcal antigen (r = .83; P less than .01), tuberculin (r = .83; P less than .01), and candida (r = .78; P less than .01). Thus, significant decreases in skin test responses and in vitro proliferation were demonstrated after therapy compared with pretreatment. Flow cytometry revealed marked increases in T-lymphocyte numbers after IL-2 alone (973 +/- 252 to 3,436 +/- 754 cells/mL; P less than .01) and IL-2 receptor-bearing cells (105 +/- 28 to 983 +/- 215; P less than .01), but not in numbers of B-lymphocytes or monocytes. Induced anergy to skin test antigens was seen during a period of relative and absolute T-lymphocyte expansion. We conclude that immunotherapy with high-dose IL-2 with or without activated lymphocytes results in a decreased response to recall antigens during a period in which lymphoid cells with nominal activation markers (Tac, DR) increase.