The sensitivity of 112 S. marcescens strains, isolated under various clinico-epidemiologic conditions, was tested by the dilution in agar method against 10 different antibiotics and sulfonamides active against Gram-negative bacteria. With the maximum concentrations used only Gentamycin and nalidixic acid were active against a high proportion of the strains tested, i.e. 96.4% and 91.1%. Kanamycin, Neomycin, Chloramphenicol, Streptomycin and Tetracyclin had an inhibitory effect against less than 25% of tested strains. One strain was resistant to all the antibiotics, 49.1% were only sensitive to 2 antibiotics and 33.9% to 3 antibiotics. Among tested strains 22 different antibiotypes were established. Evidence of 2 or more types of resistance within the same epidemic outbreaux, reduces the value of the "antibiotype" as epidemiologic indicator within this species. The readily acquired transfer factors of resistance might be responsible for the marked "mobility" of the antibiotypes, as well as for the spread of S. marcescens in hospital pathology.