S. derby K 89 isolated from sick children have been found to contain plasmids which determine the resistance of these organisms to penicillin and chloramphenicol. 3 types of plasmid DNA with molecular weights of 40. 12 and 8 megadaltons (Md) have been detected by electrophoretic analysis. The dissociation of the large plasmid (40 Md) into 3 independent replicons has been observed. The circular, covalently closed forms of the DNA cointegrate and its constituent replicons have been identified by electron microscopy. In S. derby K 89 grown in broth prepared from dried meat concentrate the loss of these plasmids is observed, and the culture thus becomes sensitive to penicillin and chloramphenicol. This suggests that dried meat concentrate contains some factor capable of eliminating the plasmids.