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Six transplanted rat tumours (three mammary carcinomas and three fibrosarcomas), all of spontaneous origin and of limited immunogenicity, have been examined for susceptibility to immunotherapy with BCG (Glaxo). Growth of limited numbers of cells from five tumours was suppressed when cells were injected subcutaneously in admixture with BCG organisms. There was no clear correlation between the immunogenicity of tumour lines and their susceptibility to regionally applied BCG. Active specific immunotherapy, using vaccines of viable or radiation-attenuated tumour cells in admixture with BCG, was reproducibly successful with only one tumour, the mammary carcinoma Sp4, this being the most immunogenic of the tumours examined. These studies indicate that naturally arising tumours are less susceptible to BCG-mediated suppression than carcinogen-induced tumours widely used for experimental immunotherapy, but indicate that local application of BCG may give the best therapeutic response.