The present study demonstrated that a human B-cell line derived from non-Hodgkin's lymphoma. HCF-MLpN. constitutively expressed G-CSF receptor on the cell surface. G-CSF binding to the cell surface was shown by immunofluorescence staining using biotinylated G-CSF preparation and analysed by flow cytometry. Specific binding of G-CSF to the cells was shown by pretreatment with unlabelled G-CSF. In the radioreceptor assay and Scatchard plot analysis using radiolabelled ligand, MLpN cells revealed a single species of binding site with an equilibrium dissociation constant of 167 (153-182) pM and a maximal binding site per cell of 1076 (1044-1116). The G-CSF receptor mRNA transcript was exhibited in the RNA from MLpN cells by reverse transcriptase polymerase chain reaction procedure. [3H]thymidine incorporation and trypan blue exclusion showed that the G-CSF receptor was capable of transducing the growth signal to HCF-MLpN cells. A small fraction of fresh B blasts from six patients with B-cell lymphoma and leukaemia displayed G-CSF binding by two-colour immunofluorescence staining. In contrast, a panel of seven B-cell lines was negative for the binding to biotinylated G-CSF preparation. These results suggest that the phenotype of G-CSF binding may be lost during the culture. The expression of G-CSF receptor in HCF-MLpN cells appeared to be exceptional.