Ten critically ill patients presenting with nosocomial infection caused by Serratia marcescens (SM) not responding to prior chemotherapy were treated in an open study with Moxalactam (MOX) alone [6] or in combination with an aminoglycoside [4]. In initial disc diffusion tests, all isolates of SM were highly susceptible to MOX. Clinically, three patients were cured and four improved. Three patients died: one from SM pneumonia, one from gangrenous cholecystitis and another from ARDS. Bacteriologically, SM were eliminated from blood cultures in all seven patients with septicemia but were recovered post mortem from the lung of one patient. In three cases with localized infection, SM were eliminated once and persisted twice. Selection of resistant SM was observed in three patients but became clinically relevant in one case only. Resistant SM strains also showed reduced susceptibility to other cephalosporins and aminoglycosides. Emergence of enterococci occurred four times, in two cases with clinical consequences. MOX is a useful drug for the treatment of SM infections, but a definite risk of selecting multiresistant SM strains and of enterococcal overgrowth must be kept in mind.