Lodoxamide ethyl, a new oral anti-allergy drug, was tested for its ability to inhibit the post-treatment adverse reaction to diethylcarbamazine (DEC) in Dirofilaria immitis-infected dogs. Lodoxamide was highly effective, as judged by the absence of both overt signs and characteristic post-DEC changes in platelet numbers, serum fibrinogen level, and serum transaminase (SGOT, SGPT) levels, in blocking the reaction in dogs with a microfilaremia level that would make them moderately reactive. Lodoxamide was less effective in blocking the reaction in dogs with microfilaremias greater than approximately 10,000 mf/ml, a level which is regularly associated with a severe post-DEC adverse reaction. Some dogs with this level of microfilaremia were fully protected, others partially protected, and others were afforded no protection. Two other agents tested, indomethacin and homochlorocyclizine, were found to have comparatively little or no blocking activity.