The antibacterial activity of pure resolved R and S epimers of temocillin was determined in a variety of in-vitro test systems, including those that allowed measurement of activity during the early period following exposure to the agents, thus minimising the effect of epimerization. In conventional agar- and broth-dilution susceptibility tests involving incubation at 37 degrees C for 18 h, little difference was evident between the activities of the individual epimers. In contrast, in other tests of antibacterial activity, such as time-kill and turbidimetric studies, the R epimer and temocillin (R/S mixture 1.8:1.0) were shown to be more rapidly bactericidal than the S epimer. Overall, the R epimer and temocillin (R/S) exhibited a similar degree of bactericidal activity in vitro.