We investigated the efficacy of a single dose of 1 g of cephazolin in reducing postoperative infections in patients undergoing 'clean' operations on the lumbar spine. In a double-blind, randomised, trial there were 21 wound or urinary infections in the 71 patients who received placebo and nine in the 70 who received cephazolin (p < 0.05). Nine of the placebo patients (12.7%) developed wound infections (complicated by bacteraemia in two) compared with three (4.3%) in the cephazolin group (p = 0.07). Hospital stay was longer for infected patients than for non-infected patients (p < 0.05). Cephazolin-resistant pathogens were isolated more frequently from patients who received cephazolin than from those who received placebo.