Pulmonary surfactant is a lipoprotein complex that lowers surface tension at the air-liquid interface of the lung and participates in pulmonary host defense. Surfactant proteins (SP), SP-A and SP-D, modulate a variety of immune cell functions, including the production of cytokines and free radicals. Previous studies showed that SP-A and SP-D inhibit lymphocyte proliferation in the presence of accessory cells. The goal of this study was to determine whether SP-A and SP-D directly suppress Th cell function. Both proteins inhibited CD3(+)/CD4(+) lymphocyte proliferation induced by PMA and ionomycin in an IL-2-independent manner. Both proteins decreased the number of cells entering the S and mitotic phases of the cell cycle. Neither SP-A nor SP-D altered cell viability, apoptosis, or secretion of IL-2, IL-4, or IFN-gamma when Th cells were treated with PMA and ionomycin. However, both proteins attenuated ionomycin-induced cytosolic free calcium ([Ca(2+) ](i)), but not thapsigargin-induced changes in [Ca(2+)](i). In summary, inhibition of T cell proliferation by SP-A and SP-D occurs via two mechanisms, an IL-2-dependent mechanism observed with accessory cell-dependent T cell mitogens and specific Ag, as well as an IL-2-independent mechanism of suppression that potentially involves attenuation of [Ca(2+)](i).