We have studied seven human monoclonal gammopathies using anti-idiotypic sera. In benign and malignant gammopathies, we have observed a similar number of B lymphocytes bearing idiotypic specificities also found on the monoclonal protein. These observations suggest that the plasma cell population is only a phenotypic expression of a tumoral event occurring in a B lymphocytes precursor which can still completely differentiate. In four myeloma patients and one benign monoclonal gammopathy, we also observed T lymphocytes bearing receptors idiotypically cross-reactive with the monoclonal protein. The values ranged from 1.8 to 8.0% within the purified T-cell population. In a first hypothesis, these T lymphocytes can belong to the tumoral clone itself. The tumoral event must occur at the level of a common precursor not yet determined to B or T pathway of differentiation. In a second hypothesis, these T lymphocytes are not cancerous but are induced by a strong perturbation of the idiotypic network, due to the enormous amount of the idiotypic B-cell tumoral subset.