Analytical isoelectric focusing of plasmid-coded beta-lactamases in polyacrylamide gels gives rise to several satellite bands in addition to one major active band (microheterogeneity). When the R plasmids are transferred by conjugation into 4 different bacterial species (Escherichia coli, Klebsiella pneumoniae, Salmonella typhi-murium, Proteus mirabilis) the position of the main band remains constant, but the number and position of the satellite bands are changed. Furthermore, the minimal inhibitory concentrations of several beta-lactamines are also changed when the plasmids are transferred between these different host strains. These facts suggest that the bacterial host might modify the physical structure and the activity of plasmid-determined beta-lactamases. They also suggest that identification of beta-lactamases and comparison of their isoelectric focusing patterns should always be carried out using the same bacterial host strain.