The pre-operative urine, pelvic urine, removed calculi and calculus washings were examined bacteriologically in 24 patients undergoing removal of intrarenal calculi. Four of seven patients with struvite calculi had an infected pre-operative midstream urine specimen and six of the seven removed calculi demonstrated significant bacterial growth. Proteus mirabilis was the commonest organism isolated. Of 17 patients with oxalate calculi only one had an infected pre-operative urine culture, but in four cases the removed stones were infected. Quantitative bacteriological culture of the stones and their washings demonstrated that infection is within the stone itself. Pre-operative urine culture failed to predict infection within the stone in 60 % of patients with infected stones. The results suggest that the presence of infected urine together with the presence of renal calculus is indication for removal of the calculus.