Rats were injected intraperitoneally with various solutions, and the local cellular responses were studied using quantitative methods. Contrary to reports in the literature, pure saline did not evoke a local neutrophilia after it was injected intraperitoneally. Under similar conditions, bacterial lipopolysaccharides proved to be highly effective. Trace doses were most effective; larger doses caused a marked diminution of the response. Furthermore, a variety of nonbacterial substances were impotent when tested under comparable conditions.