Neutrophils are one of the first cellular populations to become involved in inflammatory processes and some features of the response to inflammatory stimuli can be partially reproduced in vitro by treatment with chemotactic peptides such as N-formyl-methionyl-leucyl-phenylalanine (FMLP). Nonsteroidal antiinflammatory drugs (NSAIDs), such as indomethacin, are known to interfere in vitro with human and rat neutrophil functions and to inhibit FMLP-induced aggregation. In this article we define the scatter parameters of rat neutrophils and demonstrate that flow-cytometric analysis of these cells can be used to analyze the inhibiting action of drugs in an in vitro model of aggregation. We show, in fact, that indomethacin at 100 microM (p < 0.05) and 200 microM (p < 0.01) is able to significantly reduce rat neutrophil aggregation. These results confirm the data obtained by light transmittance aggregometry and indicate that cytometric analysis of aggregation phenomena is a technique suitable for the screening of antiaggregating drugs.