A simple, sensitive, selective and reproducible method based on agar diffusion disk assay was developed for the determination of fosmidomycin and clindamycin in human plasma and urine. A disk diffusion technique was used, essentially as previously described but utilising the assay organism Enterobacter cloacae ATCC 23355 strain to seed the agar assay plates. Calibration curves were prepared from concentration response curves in plasma (0, 1, 2.5, 5, 7.5, 10, 25, 50 ng/microl) and urine (0, 10, 25, 50, 75, 100, 250 and 500 microg/microl) were all linear with correlation coefficients better than 0.990. The precision of the method based on within-day repeatability and reproducibility (day-to-day variation) was below 5% (% coefficient of variations: %C.V.). Good accuracy was observed for both the intra-day or inter-day assays, as indicated by the minimal deviation of mean values found with measured samples from that of the theoretical values (below +/-5%). Limit of quantification (L.O.Q.) was accepted as 1 ng using 40-microl plasma or 7.5-microl urine sample. The mean recovery for fosmidomycin was greater than 99%. The method was free from interference from other commonly used antibiotics including clindamycin, carbenicillin, cephalothin, chloramphenicol, kanamycin, methicillin, penicillin, erythromycin, lincomycin, tetracycline and paromomycin. The method appears to be robust and has been applied to a pharmacokinetic study in plasma and urinary excretion of fosmidomycin in a patient with malaria following oral doses of clindamycin at 1200 mg given every 8 h for 7 days.