This study was undertaken to assess the susceptibility of pigmented and nonpigmented strains of Serratia marcescens to antibiotics and human sera, and the effect of culture filtrates from pigmented and nonpigmented of Serratia marcescens on humoral and cellular immune responses in mice to thymus-dependent and indepependent antigens. Humoral immune response was measured by hemagglutinin (HA) and hemolysin (HE) to sheep red blood cell (SRBC), and Arthus or antibody response to polyvinylpyrrolidone (PVP). The cellular immune response was measured by delayed-type hypersensitivity (DTH) determined by footpad swelling reactin to SRBC. The resistance of pigmented strains of Serratia marcescens to the bactericidal action of heat inactivated human serum was insignificantly greater than that of nonpigmented strains. However, the pigmented strains were significantly more resistant to the bactericidal action of heat-untreated human serum than that of nonpigmented strains. The clinical isolates of Serratia marcestens was also tested for their resistance to several antibiotics. There was no difference between the pigmented and non-pigmented strains in the resistance to carbenicillin. However, nonpigmented strains were more resistant to gentamicin, kanamycin and tobramycin than the pigmented strains. The intraperitoneal administration of culture filtrates from the pigmented or nonpigmented strains into mice caused enhancemented of antibody response to SRBC or PVP, and of DTH to SRBC. Besides, their enhancement of immune responses was more prominent when culture filtrate from the pigmented strains was administered.