Recombinant interferon A (50 x 10(6) U/m2 three times weekly) was given to 17 patients with SCCL and 13 patients with SQL. The minimal scheduled duration of therapy was 12 weeks. Fifteen and 11 patients, respectively, were evaluable for response. All 15 patients with SCCL showed progressive disease after a period of 2.5 weeks (median; range 1-13). One patient with SQL obtained a partial remission lasting 14 months and six patients showed no change for 14-20 weeks, while the remaining patients showed progression during the initial 12 week period. Toxicity was shown to be significant and only one patient completed therapy without dose reduction. The major cause of dose reduction was fatigue and anorexia (18 patients). Fourteen patients experienced a median weight loss of 6%. Haematological and hepatological toxicity was found in six and 19 patients, respectively. In most cases parameters of marrow and liver function were reversible in spite of continuing interferon treatment.