We have studied the characterization of granulocyte colony-stimulating factor receptor (G-CSFR) in human lymphocytes. About one-third to one-quarter of the B lymphocytes from peripheral B-cell sources displayed G-CSF binding on the two-colour immunofluorescence study. The rate of G-CSFR-expressing (G-CSFR+) B cells was higher in bone marrow and cord blood than in peripheral blood, spleen and tonsil. G-CSFR expression was greater in the surface immunoglobulin D (IgD)-positive (sIgD+) B-cell population, but scarce in the sIgD- B-cell population. In tonsil, G-CSFR+ B cells were present among the cells with naive B and germinal-centre B phenotypes, but those with memory B phenotype were rarely found on triple-colour immunofluorescence analysis. Mitogen-activated, but not resting, T lymphocytes also showed G-CSF binding. Several continuous T- and B-cell lines expressed functional G-CSFR, because the addition of G-CSF enhanced the proliferative response of these cell lines. A sequence analysis of G-CSFR mRNA isoforms obtained from the T and B cells revealed that G-CSFR was derived from class I and class IV mRNA. Our results indicated that G-CSFR was constitutively expressed on the B-cell surface and was inducible in T cells.