Synergy between cefotaxime and desacetyl-cefotaxime was evaluated against 12 strains (9 Enterobacteriaceae and 3 Bacteroides fragilis) by the chequerboard technique. A 1:1 combination of cefotaxime and desacetyl-cefotaxime was synergistic against two-thirds of the 12 strains tested. The in vitro activity of the combination was compared with that of four other beta-lactam antibiotics against 96 recent clinical isolates: 78 Enterobacteriaceae, 8 Haemophilus influenzae, 10 B. fragilis. The MICs of the combination for Gram-negative bacilli were similar to those of ceftazidime. Cefotaxime/desacetyl-cefotaxime was more active than cefotetan, cefonicid and piperacillin against Enterobacteriaceae and H. influenzae.