Inbred rats were treated with extranuclear tumour membrane fractions or 3 M KCl-solubilized extracts of two antigenically distinct 3-methylcholanthrene-induced sarcomas, Mc7 and Mc57. The rats developed tumour-specific humoral antibody responses, but were not immune to tumour challenge. Moreover, they were unresponsive to subsequent immunization with irradiated tumour grafts, this effect being immunologically specific for the tumour from which the cell-free extracts was derived. In vitro studies revealed a depressed state of cell-mediated tumour immunity in animals inoculated with cell-free tumour extracts, and this was associated with the presence of serum inhibitory factors and suppressor lymphoid cells which abrogated the cytotoxic effect of sensitized lymphoid cells in vitro. It is postulated that the development of an inappropriate immune response due to the effect of tumour antigen during the induction phase of tumour immunity may be relevant to the immunobiology of tumour-bearing hosts.