It is believed that mice do not express cutaneous basophil hypersensitivity because they have few or no basophils, We have looked for these cells in peripheral blood smears of C3H/He, DBA/2, C57BL/6, and Balb/c mice before and after a series of foreign protein injections, a treatment known to induce basophilia. We found 0.2 and 1.5% basophils in the untreated C3H/He and DBA/2 mice and no basophils in the untreated C57BL/6 and Balb/c mice. Basophilia of 2–10% was found in the treated mice of the four strains. We noticed that the microscopic appearance of the mouse basophils differs from that in other species. These results indicate that basophils may be as frequent in mice – believed not to express cutaneous basophil hypersensitivity – as they are both in man and guinea pigs – known to express this entity. Since the mouse basophils have a distinct morphology, different from that of other species, it is possible that they may not have been noticed in routine skin preparations.