The grade of neutropenia after chemotherapy seems to be correlated to the bone marrow cellularity as judged by biopsies. Prolonged blood neutropenia after sequential chemotherapy reduces dose intensity and increases the risk of severe infections. A predictive non-invasive test for marrow cellularity is needed in the attempt to predict chemotherapy-induced blood neutropenia. Thirty-one patients with haematological disorders were studied with measurements of blood absolute neutrophil counts (ANC) 24 h after a single subcutaneous injection of recombinant human granulocyte colony stimulating factor (rhG-CSF) or granulocyte-macrophage CSF (rhGM-CSF). Before cytokine administration all patients had bone marrow biopsies performed. The median increase in blood ANC 24 h after cytokine administration was 15.9 x 10(9)/l (range 3.7-34.2) in 18 patients with normo- or hypercellular marrows and only 0.4 x 10(9)/l (range 0.0-11.2) in 13 patients with hypocellular marrows (P < 0.00001). An increase in ANC or more than 5 x 10(9)/l was predictive for normo- or hypercellular bone marrows with a sensitivity and specificity of 94% and 84%, respectively. A subsequent pilot study in selected patients with prolonged neutropenia was performed. The ANC increment in 12 cases before chemotherapy correlated to the grade of neutropenia and may predict the risk of febrile neutropenia. It is suggested that blood responsiveness to myeloid growth factors correlates with marrow cellularity and may identify outpatients with risk for severe neutropenia after cyclic chemotherapy.