Cefoperazone pharmacokinetics were studied in unweaned calves. The antibiotic was administered to 10 calves intravenously, to eight calves intramuscularly at 20 mg kg-1 and to 10 calves intramuscularly at 20 mg kg-1 together with probenecid at 40 mg kg-1. Serum concentration versus time data were analysed by non-compartmental methods based on the statistical moment theory. The intravenous data were also fitted by a linear, open two-compartment model. The terminal halflife of cefoperazone was 127.9 +/- 28.2 min (mean +/- SD) after intravenous and 136.9 +/- 19.6 min after intramuscular administration. The t1/2 was increased to 257.3 +/- 127.3 min by the co-administration of probenecid. The total body clearance was 8.16 +/- 1.60 ml min-1 kg-1 and the volume of distribution at steady state was 0.713 +/- 0.167 litre kg-1. The mean residence time values were 87.2 +/- 10.6 min after intravenous and 140.3 +/- 20.6 min after intramuscular injection and were increased to 264.5 +/- 99.8 min by the co-administration of probenecid. The estimated mean absorption time was 53.1 min and the estimated bioavailability after intramuscular administration was 76.3 per cent. The minimal inhibitory concentration (MIC90) values of cefoperazone ranged from 0.5 to 2 micrograms ml-1 for Escherichia coli, salmonella groups C, D and E and Pasteurella multocida isolates. Salmonella group B strains appeared to be highly resistant to cefoperazone with MIC90 greater than 32 micrograms ml-1. There were no significant differences between the pharmacokinetic variables calculated by statistical moment theory or compartmental analysis indicating central compartment output of cefoperazone.(ABSTRACT TRUNCATED AT 250 WORDS)