Supernatant from matched, but not from unmatched, monocytes primes neutrophils for fMLP-induced oxidative burst. Human neutrophil suspensions were incubated with different supernatants from monocyte cultures preincubated with human HLA class II antibodies (matched or unmatched). After addition of fMLP, the oxidative burst activity of neutrophils was measured by flow cytometry. Pictures are representative for n = 7 separate experiments.