Eight cases of intrahospital infection by Serratia marcescens were reported from a pediatric service. The authors emphasize the predisposing factors: surgery, 'instrumentation', antibiotics and severe basic pathology. Antibiotic sensitivity, as well as biochemical properties were studied; red pigment is only a secondary and inconsistent characteristic, whose lack can lead to a mistaken diagnosis of Klebsiella Enterobacter thus masking the actual extent of Serratia's nosocomial infection.