Platelets activated with the inflammatory mediator PAF were shown to release a soluble, heat stable mediator(s) capable of causing a respiratory burst and degranulation in neutrophils, in vitro, in a dose dependent manner. The amount of superoxide production, myeloperoxidase release, and alladine phosphatase release induced by activated platelet supernatant (APS) was not significantly different (p > 0.05) from that caused opsonized zymosan particles. Morphological changes typical of activated cells were induced in adherent neutrophils by APS. The APS was equally effective whether prepared from platelet rich plasma or gel-filtered platelets, indicating that the activation was due to a platelet-derived mediator and not plasma protein activity. Since bovine neutrophils were shown to be insensitive to the thromboxane A 2 analogue U46619, and as APS from acetylsalicylic acid pre-treated platelets was equally as active as normal APS, it was concluded that neutrophil activation was not due to thromboxane release by platelets.