The age dependence of the susceptibility to passive anaphylactic shock was studied in the mouse. Anti-BPO IgE monoclonal antibody produced potent systemic sensitization sufficient for provocation of lethal shock in most aged (6 to 10 months) CTS, DS and C57BL/6J mice but only in a very few young (1.5 to 2.5 months) mice. A similar trend was found in the NOD strain, though it was not as definite as in the above three strains. Age-dependent potentiation of the IgE antibody-mediated anaphylactic shock was not found in both sexes of five other strains, C3H/He, DBA/2, NON, BALB/c and B6D2F1. However, the potentiation became obvious even in these strains, when they were treated with Bordetella pertussis before the antigen challenge. Age-dependent potentiation was also clear with IgG1 antibody-mediated anaphylactic shock in DS females and NON males. In contrast, no age-dependent difference was seen for the shock induced by PAF which is estimated to be the main mediator for anaphylactic shock in the mouse. This suggests that the age-dependent potentiation of anaphylactic shock does not seem to be due to elevated susceptibility to the mediator but to its increased release. The sex-dependent differences was also studied and found to be particularly clear in the case of IgG1 antibody-mediated anaphylactic shock in young DS and aged NON mice.