A mouse strain named ASK that was originally isolated from El (epilepsy) mice has been shown to be highly sensitive to anaphylactic shock. Here, we characterized the bases of the sensitivity of ASK mice in comparison with the parental strain, El. More than 90% of ASK mice, but not El mice that had been sensitized either actively or passively, died within 1 h following an antigen challenge. The anaphylactic death was effectively blocked by diphenhydramine. Plasma histamine levels increased by 30-50 fold in ASK after the antigen challenge, but only a 2-3-fold increase was observed in El mice. All (El x ASK) F1 mice, either male or female, showed an ASK-like phenotype, suggesting that the impaired plasma histamine response in El mice is due to some recessive mutation(s). Consistent with the plasma histamine responses, cultured mast cells derived from El bone marrow showed impaired potency to degranulate in response to surface IgE engagement, in contrast to ASK mast cells which undergo normal degranulation. Another characteristic feature of ASK mice is their sensitivity to histamine, since 75% of the mice were killed by the subcutaneous administration of 100-200 mg/kg histamine, while C3H and BALB/c mice were resistant to even 600 mg/kg histamine. Taken together, the major bases of the susceptibility to anaphylactic shock in ASK mice are thought to be the enhanced sensitivity to histamine and the recovered degranulation machinery in mast cells that is impaired in El mice.