Due to the relatively low tumour cell contamination of peripheral blood in patients with multiple myeloma, autologous transplantation of circulating stem cells may have theoretical advantages over autologous bone marrow transplantation. In four patients with multiple myeloma who where considered potential candidates for autologous stem cell transplantation G-CSF (600 micrograms/day) was administered following chemotherapy in order to maximally increase the number of circulating progenitor cells during haematopoietic rebound and to facilitate progenitor cell harvest by leukapheresis. In two previously untreated patients the administration of G-CSF following chemotherapy according to the UVA protocol (ultralan, vincristine, adriamycin) greatly increased circulating haematopoietic stem cells from 247 to 7552 CFU-GM/ml in patient 1 and from 173 to 6361 CFU-GM/ml in patient 2, which by far exceeded the increase in progenitor cells following chemotherapy alone, namely only to 594 and 317 CFU-GM/ml in patient 1 and patient 2, respectively. In two repeatedly pretreated patients, the combination of UVA and G-CSF was much less effective. Progenitor cells increased from 144 to 735 CFU-GM/ml in patient 3 and only from 222 to 232 CFU-GM/ml in patient 4. In both cases, however, mobilization of haematopoietic progenitor cells by G-CSF following cyclophosphamide (50 and 70 mg/kg body weight, respectively) led to much higher CFU-GM peak values (5324 in patient 3 and 2245 in patient 4), thus allowing an adequate harvest of mononuclear cells and CD 34+ cell numbers to achieve, in all probability, the prompt and complete reconstitution of haematopoiesis in case of transplantation.(ABSTRACT TRUNCATED AT 250 WORDS)