A semiautomatic microdilution method is described, which requires 44 μl of blood for each determination of antibacterial activity. The test strain used was Staphylococcus aureus SG 511; the medium was Standard I nutrient broth. The end point was determined after 14–16 incubation by the changing color of the indicator (phenol red). The dilution factor was alternated, 1:1.33 and 1:1.5; the standard was performed in triplicate, both leading to a lower deviation as compared with the twofold dilution methods. In a study of clindamycin pharmacokinetics in children, almost identical blood levels were obtained when comparing the microdilution method with a previously used microdisc diffusion method; however, the microtube dilution method was easier to perform.