The emergence of Serratia marcescens septicemia as a new threat in surgery is shown by the occurrence of 42 cases of this infection at the University of Cincinnati Medical Center in the seven years between 1962 and 1969. A sharp increase has been documented in 1965 and 1966. In the past the S marcescens has been generally considered to have little or no virulence. Many of the manifestations of S marcescens sepsis are similar to those associated with septicemia caused by other gram-negative organisms. Most of the cases were related to antecedent or concurrent antibiotic therapy, usually employing large dosages. The sources of infections were usually the urinary tract, thrombophlebitis at a continuous intravenous site, or the respiratory tract following respiratory assistance therapy or tracheostomy. The mortality in this series of 42 patients was 40%, indicating its seriousness in debilitated surgical patients with predisposing or preexisting diseases. Recently an increasing