It has been reported that several types of human cancer are associated with elevated levels of class-A immunoglobulins (Ig) and IgA-containing immune complexes. Moreover, most previous work has come up against the lack of IgA reactivity for chemically defined antigen (Ag). To overcome this, we first evaluated possible immunological binding in sera of patients with mammary tumors or malignant hematologic diseases and controls on a mouse monoclonal anti-idiotypic antibody (Ab2), internal image of conjugated benzo(a)pyrene (BP) coated on well plates. Using this indirect ELISA, a statistically highly significant immunological binding was found in sera of patients with mammary tumors of every grade, type and size. This immunological binding was linked to the IgA isotype. Second, we performed competition experiments between a BP conjugate coated on well-plates and anti-anti-"BP-like" antibodies (Ab) previously incubated with rabbit idiotypic antibodies (Ab1) directed against conjugated BP. A part of these anti-anti-BP-like Ab, raised in rabbits immunized with human IgA of patients with mammary tumors, recognized the Ag-combining site of polyclonal Ab1, previously developed in a rabbit immunized with BP conjugates. It appears that part of the human Ig from sera of patients with mammary tumors shares common idiotopes with rabbit polyclonal Ab1 raised against conjugated BP.