Eight Salmonella typhi-murium strains coding resistance to ampicillin were chosen from 38 strains isolated in different counties of Hungary in 1981, and their plasmids were characterized by agarose gel electrophoresis. Incompatibility groups and molecular weights of transferable R-plasmids coding resistance to ampicillin were determined and restriction enzyme analyses were done. The studies showed that among R-plasmids coding for ampicillin resistance in S. typhi-murium strains IncI alpha group plasmids with a molecular weight of 66 Mdal were dominant in Hungary. According to their cleavage patterns, the examined plasmids formed two groups. Both types contained some fragments identical in size, and they are supposed to be connected evolutionarily.