The possible involvement in vitro of 3', t'-cyclic GMP (cGMP) in the mechanism of action of the lymphokine, leukocyte migration inhibitory factor (LIF), was investigated. Partially purified LIF-rich supernatants, but not their control counterparts, induced a 2-fold increase in the cGMP levels of purified human polymorphonuclear (NMN) leukocytes. The effect was no influenced by heat-inactivated horse serum; it was manifested within 3 min of exposure to LIF and it subsided within 180 min. LIF and the supernatant factor responsible for the cGMP-generating effect were both rendered inactive by treatment with the serine esterase and protease inhibitor, phenylmethylsulfonyl fluoride, indicating that these factors are closely related, if not identical. A potent phosphodiesterase inhibitor, dipyridamole (2 x 10(-4) M), induced a 3- to 5-fold increase in PMN leukocyte cGMP levels, but combined treatment with purified LIF and dipyridamole did not add to this effect. This suggests that the cGMP-generating factor acts on the biochemical pathway that degrades cGMP.