ITERATURE on neoplasia and Rous virus neutralizing antibodies (RVNA) in avian populations was reviewed in a previous paper.8 Other information presented indicated that farm flocks experiencing significant losses associated with visceral lymphomatosis also appeared quite susceptible to the Rous virus or to an antigenically related agent on the basis of the presence of RVNA. Reported herein are further studies of this problem in breeder and experimental flocks. As in the previous paper,8 the susceptibility of a population to the Rous agent or an antigenically related agent was assessed on the presence of RVNA in the sera. Susceptibility to visceral lymphomatosis and other tumors was based on the per cent of chickens lost due to tumors determined through necropsy. Available tumor histories of the parent generations are summarized in the appropriate tables.