In September, 1981, Salmonella typhimurium infection broke out in a human infant 10 months of age on a farm located in Tochigi Prefecture. S. typhimurium strains isolated from the infant and rearing calves on the farm belonged to the same biological type 1 of Duguid and showed multiple resistance to tetracycline (Tc), chloramphenicol (Cm), streptomycin (Sm) and sulfonamide (Su). Rapid identification of plasmid DNA revealed that the S. typhimurium strains isolated harbored two kinds of plasmid DNAs in the same cell. Large plasmid DNA 98 megadaltons in molecular weight was found to be a conjugative R plasmid with resistance to 4 drugs (Tc. Cm. Sm. Su).Restriction enzyme cleavage analysis showed that R plasmids of S. typhimurium isolated from the infant and calves were indistinguishable in their cleavage patterns. This result indicates common ancestry for these R plasmids. These findings suggest that the S. typhimurium strains may have been transmitted from the diarrheal calves to the infant on the farm. The rapid identification of plasmid DNA seemed to be a useful method for obtaining a subsidiary epizootiologic marker in Salmonella infections