A total of 218 Proteus strains isolated from clinical sources were tested for their susceptibility to three penicillins and two cephalosporins. The ability to beta-lactamase production was examined in 36 of these strains. Proteus mirabilis strains were generally more susceptible to cephalosporins than to penicillins, whereas indole-positive Protei were almost uniformely resistant to cephalosporins as well as to ampicillin and benzylpenicillin but susceptible to carbenicillin. Fairly good correlation was found between the amount and hydrolytic soectryn if beta-lactamase activity and the pattern of resistance to penicillins and cephalosporins in the strains examined. Some observations indicate, however, that the resistance of Proteus bacilli to this group of antibiotics is partly related to permeability barriers in bacterial cell. About 37% of Proteus strains transferred their ampicillin resistance to E. coli K12. Beta-lactamase activities mediated by R plasmids in E. coli cultures were 1.5 to 5 times higher than in respective Proteus donors.