The relationship between mortality and the bacteraemic profile was investigated in a pneumococcal (serotype 6B) sepsis BALB/c mouse model where animals received protection by specific hyperimmune serum. A single intraperitoneal dose of hyperimmune serum obtained from mice immunized with the heat-inactivated strain was administered (non-diluted or diluted to 1/4 or to 1/16) to 5-mice study groups 1 h prior to intraperitoneal inoculation with the infective inoculum (3.57 x 108 cfu/ml). Blood cultures were performed daily over 15 days, with 8 microl of blood being collected from the tail vein; the samples were resuspended in Todd-Hewitt broth containing 10% trisodium citrate and plated onto blood agar for colony counting. Animals included in the control group received placebo (PBS). Mortality was 100% in control animals within the first 48 h. Hyperimmune serum decreased and delayed mortality in a dose-related trend, producing 100%, 80%, 60% and 40% survival rates at 72, 96, 144 and 360 h, with non-diluted serum. Bacteraemic profiles with maximum colony counts > or =5 x 107 cfu/ml in blood during the follow-up period were related to > or =65% probability of death, regardless of the serum dilution administered.