In 4 experiments, strains of chickens relatively susceptible (S-strain, P-line) or resistant (N-line, PDRC) to Marek's disease (MD) were challenged with Rous sarcoma virus (RSV) at 6-8 weeks old. RSV tumors occurred in 94-100% of the birds in each strain, but the number with tumors that regressed during the 40-46-day experiment varied from 7% (S-strain) to 91% (PDRC). The N-line and P-line birds, derived from a random-bred flock with selection only for susceptibility or resistance to MD, regressed their tumors at about equal rates (respectively 59 and 65%). Thus, although the genetic strains differed in ability to regress RSV tumors, the difference was not necessarily related to genetic resistance or susceptibility to MD. Other birds in 2 of the experiments, infected 2-4 weeks earlier with JM or GA isolants of MD virus, had significantly (P less than 0.01) lower incidence of RSV tumor regression than did controls provided that they developed gross lesions of MD.