We examined platelet-activating factor (PAF)-like activity in eosinophils obtained from asthmatic and nonasthmatic patients. PAF-like activity was detected by aggregation of washed guinea pig platelets. Eosinophils from asthmatic patients stimulated with C5a (a human complement factor), N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP), or calcium ionophore A23187 had a mean activity of 8.86 ± 1.99 ng/10 6 cells in the supernatants and 20.34 ± 7.11 ng/10 6 cells in the cell pellets, 10.75 ± 0.78 and 25.44 ± 5.23, and 12.89 ± 1.74 and 36.67 ± 6.43, respectively. Mean PAF-like activities in eosinophils from nonasthmatic patients stimulated with C5a, fMLP, or A23187 were 5.91 ± 0.23 ng/10 6 cells in the supernatants and 9.12± 1.95 ng/10 6 cells in the cell pellets, 6.99 ± 0.93 and 7.09 ± 1.11, and 7.22 ± 1.42 and 9.78 ± 2.03, respectively. With no stimulation, mean PAF-like activities in both eosinophil populations were under the detection limit in the supernatants and in the cell pellets. PAF-like activity was greater in asthmatic eosinophils than in nonasthmatic eosinophils after stimulation with C5a, fMLP, or A23187, suggesting that asthmatic eosinophils contribute to the development of bronchial asthma by inducing more PAF-like activity.