Inbred rats were injected s.c. with cells of syngeneic hepatoma D23, D23 cells + BCG as a mixed inoculum, mixed inoculum one side and D23 alone contralaterally, or BCG alone. Their blood mononuclear cells were tested weekly for cytotoxicity against D23 target cells using a microcytotoxicity method and their serum was tested for blocking activity against cytotoxicity by lymph node cells from immunized rats. Tumour growth was suppressed when BCG was in contact with tumour cells but tumours grew unhindered if the BCG was given contralaterally. All rats receiving tumour cells, either alone or mixed with BCG, developed cell-mediated cytotoxicity which remained until termination at 35 days. Rats receiving BCG alone showed slight initial cytotoxicity which disappeared after 7 days. Blocking factors appeared in the serum of rats which developed growing tumours but not in rats whose tumours were suppressed by contact with BCG. Splenectomized rats did not differ markedly from intact rats in the in vitro studies or in vivo. It is concluded that development of cell-mediated immunity and blocking factors depends more upon treatment with tumour cells and the subsequent behaviour of the tumour than upon treatment with BCG per se.