It is the intention of this communication to present a simple and reliable in vitro system for the evaluation of anti-inflammatory drugs. The system described appears to combine the advantages of two established procedures, i.e. measurement of the inhibition of prostaglandin (PG) synthesis in vitro using microsomal enzyme preparations and the measurement of the inhibitory effects of drugs on PG synthesis in vivo avoiding major limitations. We propose to measure the effect of drugs on the PG release from mouse peritoneal macrophages in vitro. The influence of culture conditions, phagocytosis and drugs on the PG release from these cells is described. We found that antibody-coated erythrocytes are especially suitable triggers of PG release, and that the release can be inhibited by steroidal and non-steroidal anti-inflammatory drugs in concentrations resembling their effective doses in vivo.