Infection of mouse embryos with a variant type of murine leukemia virus (MuLV) results in a high percentage of thymic lymphomas which die in a short period after birth. The few animals which survive are persistently viremic for a period of years. These animals frequently develop tumors at other sites. This process can be mimicked by injection of fetal or newborn thymocytes into irradiated syngeneic hosts or by injecting newborn thymocytes, T cells or mature T cell lines into thymectomized adult hosts. Fetal and newborn thymocytes will be used as the source of target cells for the study of this process. Infected adult (C57B1/6 X BALB/c)F1 hybrids will be used as source of infectious virus. Neonates (BALB/c X A)F1 will be used as source of T cells to test for helper activity in the virus producing cells. In this latter system there is no involvement of thymic T cells which are normally required for the process of helper activity by adult T cells. The infective virus and a high molecular weight fraction from crude tissue culture extracts will be tested for their ability to induce the disease by infecting developing and adult thymocytes. A high molecular weight fraction from crude tissue culture extracts has been isolated which has been implicated in oncogenic transformation in tissue culture and possibly in development of lymphoma in MuLV infected animals. It is possible that this fraction is responsible for the induction of neoplasia by the virus. Some fraction of it will be isolated from fetal or newborn thymocytes in order to determine if it has a role in the induction of lymphoma in MuLV infected animals.