The hyporesponsive state of lung-derived mononuclear leukocytes has been, in part, attributed to the effects of the lipid rather than the protein components of pulmonary surfactant. In the present study, however, the results suggest that purified preparations of pulmonary surfactant-associated protein A (SP-A) suppress both phytohemagglutinin (PHA, 1 microgram/ml)- and anti-CD-3 (1 to 10 ng/ml) activated proliferation of human peripheral blood and tonsillar mononuclear cells in a dose-dependent manner at concentrations as low as 50 pM (6.25 micrograms/ml) when added at the initiation of cultures. Addition of SP-A to PHA-stimulated peripheral blood mononuclear cells (PBMC) as late as 24 to 36 h after PHA was also capable of suppressing [3H]thymidine incorporation measured at 72 h. In contrast, concanavalin A (Con A; 2 micrograms/ml)-stimulated PBMC proliferation was slightly augmented by the addition of SP-A. Analysis of the supernatants of PHA-stimulated cultures treated with SP-A revealed that accompanying the inhibition of proliferation was a corresponding decline in measurable interleukin-2 (IL-2) concentrations, from 154 pg/ml for the PHA-treated cells to 57.8, 28.4, 5.2, and less than 2 pg/ml of IL-2 when SP-A was added at 6.25, 12.5, 25, and 50 micrograms/ml, respectively. We suggest that the action of SP-A on PHA-stimulated human PBMC may involve the blocking of a costimulatory signal crucial for in vitro T-cell activation.