The ubiquitous nononcogenic lactic dehydrogenase RNA virus (LDV) is one of the most common contaminants of transplantable tumors and oncogenic viral preparations. This association of LDV with a large variety of murine tumors and the known effect of LDV infection on immune competence may suggest the existence of a possible etiological relationship between the virus and murine tumors. We have found that the inoculation of transplantable carcinoma cells to acutely LDV-infected mice is followed by an enhanced rate of tumor growth. To examine the possibility that LDV is involved in the carcinogenic process and may act as a promoter or initiator, we infected mice with LDV at various time intervals prior to, simultaneously with or after their treatment with chemical carcinogens (urethan 3-methylcholanthrene, and 9,10-dimethyl-1,2-benzanthracene) and then followed the development of the malignant disease. No differences in the rate of tumor appearance and in mortality were observed between LDV-infected and uninfected mice. In addition, LDV failed to affect the development of spontaneous reticulum cell sarcoma and lymphatic leukemia in SJL/J, AKR/Cu, and AKR/J mouse strains. It is suggested that, although LDV does not affect the induction of oncogenic processes, this virus may still be capable of promoting the progression of transplantable tumors due to its immunoregulatory properties.