Reduction in the dosage of dicloxacillin from 500 mg to 250 mg 3 times a day would mean lowering of costs and less side-effects in orthopaedic infections. In this cross-over study, the serum concentrations of dicloxacillin were measured in 9 patients after administration of dicloxacillin 500 mg 3 times a day (dicloxacillin 500 mg) and after co-administration of 250 mg dicloxacillin and 250 mg probenecid 3 times per day (dicloxacillin 250 mg+probenecid 250 mg). Concentrations were measured every hour after the tablet intake. The mean maximum serum concentrations of dicloxacillin were 17.1 micrograms/ml (dicloxacillin 500 mg) and 12.2 micrograms/ml (dicloxacillin 250 mg+probenecid 250 mg), respectively (P < 0.05). Serum concentrations above 3 micrograms/ml were obtained during 285 min. in both regimes, but the individual variations were biggest during in the dicloxacillin 250 mg+probenecid 250 mg treatment. Serum concentrations above 5 micrograms/ml were in mean measured during 228 min. (dicloxacillin 500 mg) and 190 min. (dicloxacillin 250 mg+probenecid 250 mg), respectively (P < 0.05). The clinical significance of these findings is being discussed. In theory, treatment with dicloxacillin 250 mg+probenecid 250 mg may be as sufficient as dicloxacillin 500 mg.