The signs and treatment of abnormal reactions following the use of organophosphorus systemic warble fly dressings are recorded. A review is provided of all complaints received by the manufacturers and distributors of such compounds during the dressing periods for the years 1975 to 1978. The volume of warble fly dressing sold each year was known and the number of cattle dressed was based on the calculation of the number of adults which could have been treated with the volume. Complaints were divided according to the main signs reported and the most common complaint was that of paralysis-incoordination. The incidence of death or slaughter following use of an organophosphorus compound was one in 93,000 animals treated. The majority of animals showing abnormal signs other than paralysis-incoordination were involved in incidences of skin lesions, depressed milk yield, bloat or 'other reactions'. Compared with the number of animals treated, the level of reported reaction was low (one complaint per 46,000 animals treated).