Abstract
Administration of BCG at the time of rat polyoma tumor isografting or 2 weeks previously, inhibited tumor growth and induced an increased level of lymphocyte cytotoxicity and an increase in the number of circulating lymphocytes, i.e. it resulted in an increased level of cell‐mediated immunity. A similar inoculation of BCG at the time when the tumor isograft had already grown out to a palpable nodule did not inhibit tumor growth, but rather caused an enhanced growth. It did not increase the level of cell‐mediated immunity under these conditions and did not decrease the blocking activity in serum, but rather increased it. Furthermore, it did not induce any production of cytotoxic antibodies detectable with rat complement. BCG treatment of rats which had the same day undergone incomplete tumor excision increased the growth of remaining tumor and increased the blocking activity in serum. On the other hand, BCG treatment of analogous rats which had received “unblocking treatment” in the form of splenectomy and inoculation of unblocking serum caused tumor regression in some animals and increased the cell‐mediated immunity in all animals tested.