We have tested for production of caseinolytic activity the strain of E. coli K12 J62 carrying different R factors from various Enterobacteriaceae of clinical isolates. The results obtained showed the presence of a caseinolytic activity in 7 out of 139 R factors tested. All of these positive (cas+) R factors derived from strains of Klebsiella pneumoniae and all were able to transfer a strong lac+ phenotype as well as multiple antibiotic resistances (R-lac factors). Another strain of E. coli K12, J53, which is plasmid-free, produced levels of caseinolytic activity comparable to those of the Klebsiella R factors; J62, on the contrary, appeared to possess an inhibitor specific for the caseinolytic activities of J53 and of the R factors but not of other cas+ Gram+ and Gram- organisms. The hypothesis that the expression of the cas+ phenotype in E. coli K12 may be due to the presence of plasmid-linked regulatory genes is discussed.