Addition of platelet-activating factor (PAF) to human eosinophils leads to the modulation of eosinophil responses. The respiratory burst, induced by opsonized particles, consists of an initiation and a propagation phase and is greatly enhanced ("primed") after pretreatment with PAF. This priming event induces the following changes in signal transduction between the opsonin receptors (in particular the CR3 receptor) and activation of the respiratory burst: 1) an enhanced activation of protein kinase C (PK-C): the initiation of the respiratory burst in untreated eosinophils is not sensitive to PK-C inhibition (via staurosporine) and is not accompanied by accumulation of diglycerides and changes in [Ca2+]i. After pretreatment with PAF, the initiation of the response is partly sensitive to inhibition of PK-C (via staurosporine) and is accompanied by accumulation of diglycerides and a fast and sustained increase in [Ca2+]i; and 2) an enhancement of a PK-C-independent initiation of the respiratory burst. The propagation phase in both primed and unprimed cells is sensitive for inhibition by staurosporine. Our results indicate that in eosinophils the phospholipase(s) responsible for the accumulation of the diglycerides and changes in [Ca2+]i during the initiation phase of the serum-treated zymosan response seem(s) to become associated with the signal transduction route only after priming with PAF. This results in the occurrence of two signal transduction routes that can act independently of each other.