The Ig idiotype of B-cell lymphoma can be used as a tumor-specific target. Prior trials with monoclonal anti-idiotype antibodies alone and combined with alpha-interferon have shown significant antitumor activity. In some patients, idiotype-negative tumors emerged after treatment. In this trial, patients with relapsed non-Hodgkin's lymphoma were treated with two identical courses of monoclonal anti-idiotype anti-body therapy. Concurrent with the second course, at a time when idiotype-negative cells were suspected to be proliferating, a pulse dose of chlorambucil was administered. Tumor biopsies obtained before the first and second courses of treatment and at relapse were analyzed for idiotype expression and proliferation. Thirteen patients received 24 courses of antibody with minimal toxicity. Eleven had tumor regression, with 1 complete remission, 8 partial remissions, and 2 minor remissions, with freedom from progression lasting a median of 7 months in responding patients. Idiotype-negative tumor cells appeared in some relapse specimens despite the use of chlorambucil. In retrospect, this was not surprising because there was no increase in the proliferative rate of these tumors at the time the drug was used. Anti-idiotype antibodies continue to demonstrate antitumor activity against B-cell lymphoma with minimal toxicity. The mechanism of the effect is presumed to involve both direct antiproliferative effects of the antibody on the tumor cells as well as indirect, more long-lasting effects on the host. The addition of a mild chemotherapeutic agent in the dose and schedule used here to the second cycle of antibody therapy did not interfere with the antitumor effect, nor did it decrease the emergence of idiotype-negative cells.