Hairy-cell leukemia has been shown to be extraordinary sensitive to treatment with alpha-interferon. In order to define clinically effective interferon doses associated with minimal toxicity two different dose regimens were applied in this clinical trial: firstly, a conventional dose schedule, and secondly, a biologically defined dose regimen. For dose finding in the latter group, neopterin, a GTP degradation product produced by macrophages under control of interferon, was chosen. Six patients (Group A) received conventional doses of recombinant interferon--alpha-2 (rIFN-alpha-2) 3 X 10(6) U/sqm/daily by the subcutaneous route. Five patients (Group B) were treated with the minimal dose of rIFN-alpha-2 which had previously been shown to induce maximum neopterin levels in urine. Already interferon doses in the range of 3 to 5 X 10(5) U/sqm2/daily administered subcutaneously proved to be sufficient for triggering maximum neopterin excretion in the urine. After six months of interferon treatment all patients were evaluable. At this time both doses regimens proved to be effective in terms of their anti-leukemic activity, but differed significantly in toxicity, which was only seen in Group A patients.