Equine neutrophils (PMN) were isolated from citrated normal blood by density gradient separation on Ficoll-Hypaque to greater than 96% purity and 98% viability and an average of 3.78 x 10(7) PMN/ml. The agonist C5a des Arg was used in serial dilutions of whole zymosan-activated equine plasma (ZAP) or was partially purified from ZAP by column chromatography. Purified equine PMN exhibited rapid aggregation following incubation with C5a des Arg which was further dependent on the availability of divalent cations, especially Mg++. The microfilament disruptive agent cytochalasin B (5 micrograms/50 microliters) greatly augmented aggregation responses to C5a des Arg. Subaggregating doses of C5a des Arg promoted PMN adhesiveness as assayed on 0.5 x 10 cm borosilicate glass columns containing a 2.0 cm bed of Sephadex G-25. This C5a des Arg-induced increased adhesiveness was inhibitable by prior incubation of the PMN with either non-steroidal (0.065 M phenylbutazone) or steroidal (0.005 M dexamethasone) anti-inflammatory agents. Ultrastructural studies correlated well with functional assays and revealed marked organelle-free lamellipodia formation without PMN-PMN contact at subaggregating doses of the agonist and progressive PMN-PMN contact at aggregating doses. Equine PMN are responsive to C5a des Arg, and induced adhesiveness responses can be manipulated by anti-inflammatory agents.