Synergy between a 2:1 combination of ampicillin and amdinocillin was studied with 10 urinary isolates of Enterobacteriaceae. Synergy was observed with six organisms in a bladder model that reproduced many features of the milieu of the human bladder. Antibiotic susceptibility of these organisms was also determined by the agar incorporation plate method, by tube dilution (urine and broth), microtiter method (urine and broth) and disk susceptibility test. Minimal inhibitory concentrations of individual antibiotics were of no value in predicting the synergistic response and were sometimes misleading. None of the laboratory tests could predict synergy in the bladder model. Evidence is available that the bladder model may be superior to other in vitro tests for synergy studies.