A newly described 6beta-amidinopenicillanic acid derivative, mecillinam (formerly called FL 1060), showed a high in vitro activity against Enterobacteriaceae. The effect on Escherichia coli was bactericidal and was due to lysis of the cells. The longer the culture grew under the influence of mecillinam or the lower the inoculum, the greater the bactericidal effect. The morphology of the cells changed towards large spheric forms (2 to 5 mum) under the influence of mecillinam. Consequently a great discrepancy between the optical density and the viable count was seen. The morphologically abnormal cells could be protected against lysis in vitro by addition of ionized compounds such as sodium chloride. Abnormal cells were more sensitive to ampicillin than normal cells. As expected synergy could be demonstrated between mecillinam and ampicillin. This was marked under experimental conditions where the abnormal cells were protected against lysis.