G-CSF was applied in three patients with acute, iatrogenic, immunological agranulocytosis (after ticlopidine, thimazol and aminoglutethimide) complicated by severe infections. Before this treatment was started no improvement had been achieved despite the administration of antibiotics, and corticosteroids for 4 to 9 days. Two patients had anaemia and one--thrombocytopenia probably due to the damage to the earlier, common progenitor cells. In bone marrow smears a plasmocytic reaction reaching 11-13% of total cell counts was observed. After G-CSF all patients showed a prompt amelioration of clinical symptoms, and the leucocyte count raised in several days up to 11.0-73.0 x 10(3)/microliters. Simultaneously young cellular forms of granulocyte lineage appeared in peripheral blood.