Six reported cases of anaphylactic reactions due to measles vaccinations of children were investigated. The reactions appeared to be immediate hypersensitivity reactions most probably due to allergy. Trace amounts of calf serum proteins were demonstrated in the measles vaccine by means of crossed immunoelectrophoresis whereas egg antigens could not be detected by this method. The reason for the untoward reactions was not identified. Atopic traits were not reported in any of the patients but were found in first degree family members of three of them. Skin prick testing with measles vaccine was positive in one of the patients only, whereas skin prick testing with egg yolk, egg white, cow's milk and calf serum were negative in all cases. RAST was performed with allergosorbents containing different material used during the vaccine production, but was negative in all cases. The results with control individuals were in accord with previous reports that patients with allergy to hen's egg tolerate the injection of measles vaccine. Immediate hypersensitivity reactions to measles vaccine may occur independent of allergy to egg and even in children without any known allergy or atopic heredity.