In an attempt to determine the susceptibility breakpoints for amoxycillin, co-amoxiclav and cefotaxime in pneumococcal pneumonia, a neutropenic mouse model was established and tested with two strains having different susceptibility to penicillins and cefotaxime. With a penicillin-sensitive strain (MIC/MBC = 0.01/0.01 mg/L) the minimum dosage tested achieving significant cure was 2 mg/kg for amoxycillin, co-amoxiclav and cefotaxime. For the penicillin-insensitive strain (MIC/MBC = 1/2 mg/L), the minimum dosage tested giving significant cure was 50 mg/kg for amoxycillin and co-amoxiclav but 100 mg/kg for cefotaxime. Our results support the belief that MICs of amoxycillin, co-amoxiclav and cefotaxime for pneumococcal strains of < or = 0.5 or < or = 1 mg/L can be considered as clinically relevant susceptibility breakpoints.