Fifty-nine patients with severe urinary tract infections were treated with either cefotaxime or ampicillin plus netilmicin in a controlled, open randomised study of the clinical and bacteriological effects. The patients responded favourably in both groups. The minimum inhibitory concentrations of cefotaxime against the isolates from blood were low for all bacterial strains except one (Streptococcus faecalis). Time to normalisation of temperature was significantly shorter in the cefotaxime group. The results suggest that cefotaxime is an effective and well-tolerated agent in the treatment of serious infections. However, the difference between the two groups was too small to allow preference of one procedure over the other.