The objective of this study was to determine the effects of nitric oxide (NO) on lymphocyte proliferation and cytokine release. Bronchoalveolar lavage (BAL) cells served as the source of NO and were obtained from rats treated with a single, intratracheal dose of bleomycin (3.6 mg/kg). At the time of sacrifice, the spleens were removed and the lymphocytes separated. Co-cultures containing BAL cells, lymphocytes and concanavalin-A were established and incubated at 37 degrees C for 24 hours at which time proliferation, nitrite concentration and interleukin-2 (IL-2) production were measured. At ratios from 5:1 to 1:4 (BAL:lymphocyte) there was a significant reduction in lymphocyte proliferation. There was a significant, negative correlation between NO concentration and thymidine incorporation which was reversed when the NO synthase inhibitor NG-monomethyl-L-arginine (NMA) was added to the co-cultures. Despite marked inhibition of spleen lymphocyte proliferation by NO, released by BAL cells, there was no corresponding reduction in IL-2 production. These data demonstrate that macrophages, activated in vivo, produce NO which regulates lymphocyte growth but not necessarily functions such as the secretion of the cytokine IL-2. Further, the ability of IL-2-dependent CTLL-2 cells to proliferate in the presence of excess IL-2 was also inhibited by BAL cells, confirming that NO inhibits lymphocyte growth.