Animal models to control the serious neurological complications after vaccination against whooping cough are not available. In a recent paper pertussis vaccine induced acute encephalopathy in certain mouse strains (1). Healthy BALB/c mice died with shock-like symptoms after immunization with bovine serum albumin (BSA) and heat-killed pertussis. Mice not sensitized with BSA survived, and mice of strains with another H-2 type than H-2d were not susceptible. The authors concluded that the susceptibility to side effects to pertussis vaccine in mice and possibly in human is linked to the MHC. We tried to repeat the experiments reported by Steinman et al. in the hope that the murine encephalopathy model would be useful to evaluate possible neurological complications. In spite of having the same H-2d genotype, the BALB/c mice of two breeding stocks did not develop shock-like symptoms with fatal consequences after the last injection with BSA. This fact corresponds possibly with the author's observation that the pertussis vaccine encephalopathy is not under the control of H-2 genes alone. As shown in our tests the sudden deaths and encephalopathy in mice are not linked to BSA-sensitization because mice who received pertussis vaccine only showed the same symptoms as mice injected with BSA and vaccine. Histology did not indicate brain damage. It seems obvious that the deaths in our experiments were caused by the pertussis toxins present in the large numbers of bacteria given.