Human recombinant interleukin-7 (IL-7) was labeled with biotin and used to examine IL-7 receptor (IL-7R) expression and regulation on human primary hematopoietic cells, the monocytoid line THP1, and a range of B- and pre-B-celi lines by flow cytometry. A strong intensity of staining was observed using relatively high (greater than 1 x 10(-7) mol/L) concentrations of biotinylated IL-7 on the majority of cell types examined. This reactivity, which could be effectively competed with excess unlabeled IL-7, did not correlate with either mRNA levels for the cloned receptor or with estimates of IL-7R expression determined by [125I]IL-7 binding. Staining of cells with a titration of biotinylated IL-7 showed, at concentrations greater than 1 x 10(-7) mol/L binding with a Ka in the range of 1 x 10(6) mol/L-1, to 1 x 10(7) mol/L-1, an affinity 100 to 1,000 times lower than that reported for the cloned IL- 7 receptor. Further data suggesting the existence of a distinct low- affinity IL-7R were provided by two antibodies specific for the cloned IL-7R. Staining with these monoclonal antibodies (MoAbs) correlated with both IL-7R mRNA levels and receptor expression determined by [125I]IL-7 binding, but was not compatible with the distribution of reactivity seen with biotinylated IL-7. Using tritiated biotin to label IL-7, it was estimated that the total number of IL-7 binding sites on the cell lines examined ranged from 1 x 10(4) to at least 5 x 10(5)/cell. Cross-linking studies showed that [125I]IL-7 associated with two major proteins of approximately 62 Kd and 70 Kd on the surface of RPMI 1788 and THP1 cells, in contrast to the 75- to 80 Kd molecule characteristic of the previously cloned receptor, expressed on the surface of Daudi cells. Proliferation of THP1 cells, expressing only the low-affinity form of IL-7R and lacking detectable IL-7R mRNA, could be inhibited by the addition of IL-7 in a concentration-dependent fashion, indicating that, at least on this cell line, binding of IL-7 with a Ka of 1 x 10(6) mol/L-1 to 1 x 10(7) mol/L-1 can transduce a biological signal. Taken together, the data contained in this report demonstrate the existence of a low-affinity IL-7R, expressed in high numbers on hematopoietic cells of different lineages, which is the product of a gene distinct from that encoding the cloned IL-7R.