We investigated the effects of platelet-activating factor (PAF) on guinea pig peritoneal macrophages. Specific and high-affinity binding sites for PAF were detected on guinea pig peritoneal macrophages. Scatchard analysis of PAF binding revealed high affinity binding sites (7.9 x 10(4)/cell) with a dissociation constant of 2.3 x 10(-10) M. When treated with 10(-9)-10(-5) M PAF, guinea pig peritoneal macrophages released hydrogen peroxide into the medium in a time-dependent manner. The release reaction upon stimulation with 10(-5) M PAF reached a plateau within 30 min and the extent of release was twice as high as that when stimulated by N-formyl-L-methionyl-leucyl-L-phenylalanine (fMLP; 2 microM)-treated cells. Neither lysoPAF nor the PAF enantiomer was effective. PAF-induced H2O2 release was inhibited specifically by PAF antagonists, suggesting that PAF activated macrophages through binding to specific sites. Lysosomal enzyme (N-acetyl-beta-D-glucosaminidase) was released from guinea pig peritoneal macrophages upon treatment with 10(-5) M PAF for 60 min. Guinea pig peritoneal macrophages were treated with PAF for 8 hr and the conditioned medium was examined for cytokines. The medium exhibited cytocidal activity against mouse fibroblast L929 cells [tumor necrosis factor (TNF) activity], and this activity was comparable to that detected after treatment of cells with the bacterial lipopolysaccharide (LPS). Furthermore, the same conditioned medium also showed colony-stimulating factor (CSF) activity. Generation of these cytokines was stereospecific. Our findings suggest that PAF is a unique macrophage activator that potentiates both respiratory burst/lysosomal enzyme release (early-phase response) and monokine production/glucose consumption (late-phase response).