Sera from rats bearing Morris hepatoma 3924A for 7 weeks contained antibodies against the 48 kd tumor-type poly(A) polymerase, whereas sera from normal rats or rats bearing the tumor for shorter periods did not exhibit immunoreactivity against the enzyme. Purified IgG from the sera of the tumor-bearing rats inhibited both cleavage at the poly(A) site and polyadenylation of adenovirus L3 RNA in nuclear extracts derived from HeLa cells. By contrast, IgG from normal rats did not block the 3'-terminal processing reaction. Control or immune IgG had no effect on the transcription of ribosomal gene in the extracts derived from H-4 hepatoma cells. These data demonstrate the functional specificity of the anti-poly(A) polymerase antibodies found in the sera of the tumor-bearing rats.