Broiler chickens free of maternal immunity to reticuloendotheliosis virus (REV) were used in the experiment. Two groups of 25 chickens were inoculated with REV at one day of age. One of these groups and another group of 25 chickens were inoculated with Eimeria tenella sporulated oocysts at 7 days of age. Chickens inoculated with E. tenella showed bloody diarrhoea from 12 to 14 days of age. Six out of 25 chickens died (P less than 0.05) at 13 and 14 days of age in the dual infected group. At 14 days of age, when chickens were killed, the lesion score in the combined infection group, was statistically different from that in the chickens inoculated with E. tenella alone. Also the weights of the bursa of Fabricius and thymus were lower in the two REV infected groups than in the controls. Although REV infection alone adversely affected the weight gain and feed conversion, with combined infection this effect was much greater. Following REV inoculation most of the chickens showed feathering defects and all the examined chickens were viraemic at 21 days of age. At the same age, all but one chicken failed to show precipitating antigenaemia and about one-half of these chickens showed a very low serum neutralisation titre. None of these chickens showed precipitating antibodies.