In this report a simple procedure for the production of murine monoclonal antibodies (MoAb) against the idiotype of malignant B cells is described. Mice were immunized with lymphoid cells from patients with B-cell chronic lymphocytic leukaemia (B-CLL). After fusion of the spleen cells, hybridoma supernatants were screened for anti-idiotypic MoAb in ELISA with immunoglobulins obtained from tumour-cell lysates, xenohybridomas and patients' sera. The anti-idiotypic MoAb were used to study tumour cells and serum immunoglobulins (Ig) from four different patients with B-CLL. It was found that the serum IgM and IgD in one patient shared the same idiotype. Evidence is presented that IgG-secreting cell populations are not restricted to lambda-Ig-light chain-expressing B-CLL cells. With the help of anti-idiotype MoAb accurate measurements of total and idiotype-positive serum immunoglobulin levels during chemotherapy were possible.