To ensure prolonged survival, dogs with cyclic neutropenia should be protected against bacterial infection of exogenous or endogenous origin, particularly during the neutropenic episodes. One of the methods available to minimize the risk of infection in these dogs, is selective decontamination of the gastrointestinal tract by using antibiotics and/or chemotherapeutic agents, in conjunction with housing in a laminar-flow cabinet. Two pregnant bitches, some of the offspring of which were expected to be homozygous for the cyclic neutropenia allele, were decontaminated with nalidixic acid. Fourteen days after initiation of the antibacterial treatment, the two dogs died. Jaundice and seizures had been apparent in both animals prior to death. Histopathological examination revealed changes primarily in the liver, which were consistent with toxic hepatic necrosis and were characterized by severe centrilobular haemorrhage and disappearance of hepatocytes. Multiple haemorrhages were observed in other organs. Further clinical investigation in two other dogs strongly suggested that nalidixic acid was the cause of death in the two pregnant bitches.