Abstract
R factor-determined .beta.-lactamases [Escherichia coli] were investigated by analytical isoelectric focusing. The enzymes such as those specified by the R6K and RP4 plasmids (TEM-type enzymes) are notably homogenous in biochemical tests but 2 subclasses can be distinguished by isoelectric focusing. Three subclasses can be distinguished among the oxacillin-hydrolyzing enzymes, in good agreement with the classification based upon biochemical characteristics. The TEM-type .beta.-lactamases are promiscuously distributed among plasmids of a wide variety of compatibility groups, whereas the various oxacillin-hydrolyzing enzymes show some degree of correlation with compatibility.