Some anaesthetics are known to depress granulocyte chemiluminescence responses in the phagocytosis of zymosan as a measure of their oxidative microbicidal capacity. In this study the effects of thiopental were measured on chemiluminescence responses in the phagocytosis of Staphylococcus aureus and Escherichia coli, which are the commonest bacteria causing postoperative infections. Granulocytes from 14 healthy volunteers (mean age 35 +/- 9 (s.d.) years) were tested in vitro in the presence of clinical thiopental concentrations 0, 5, 10 and 25 micrograms/ml (0, 18, 36 and 91 mumol/l, respectively). During a 60-min exposure period, decreased chemiluminescence responses occurred to Staph. aureus at 5 micrograms/ml (P less than 0.01) and higher thiopental concentrations and to E. coli at 10 micrograms/ml (P less than 0.01) and higher concentrations, as a sign of depressed oxidative microbicidal activity.