Thirteen clinical isolates of Plesiomonas shigelloides were tested for antibiotic susceptibility of tetracycline, kanamycin-sulphate, gentamicin, erythromycin and ciprofloxacin. All the strains were susceptible to all these antibiotics. The minimal inhibitory concentration (MIC) of all these antibiotics were: Kanamycin-sulphate-MIC 0.4-10.0 micrograms/ml, tetracycline-MIC 0.25-2.4 micrograms/ml, gentamicin-MIC 2.0-12.0 micrograms/ml, erythromycin-MIC 0.5-14.0 micrograms/ml and ciprofloxacin-MIC < or = 0.006 micrograms/ml. The MIC and MBC (minimal bactericidal concentration) of the antibiotics kanamycin-sulphate, tetracycline, gentamicin, erythromycin and ciprofloxacin changed little in the presence of serum protein. Different concentrations of sub-MICs played synergistic role in serum bactericidal action on this organism.