Hemopoietic cells have been reported to synthesize insulin-like growth factor-I (IGF-I) messenger RNA (mRNA), but the relative contribution of specific cell lineages that express these transcripts remains unknown. Reverse transcription and amplification of complementary DNA (cDNA) by the polymerase chain reaction were used to characterize full-length IGF-I mRNA transcripts in murine hemopoietic cells. The identity of transcripts encoding the entire prepropeptide was confirmed by restriction endonuclease digestion, Southern blotting, cloning, and Sanger sequencing. Abundance of IGF-I mRNA transcripts was assessed both by Northern blotting and sensitive ribonuclease protection assays followed by quantification with Phosphor-Imager analysis. Whereas IGF-I cDNA transcripts could be detected in a variety of leukocytes after polymerase chain reaction amplification, IGF-I mRNA was negligible or nondetectable in T and B cell lines and in those tissues containing a predominance of these cell types (e.g. spleen and thymus) by Northern blotting and ribonuclease protection assays. In contrast, elicited peritoneal macrophages, a macrophage cell line, microglia, and bone marrow macrophages differentiated in vitro expressed abundant IGF-I mRNA transcripts, whereas neither a premyeloid cell line nor freshly isolated bone marrow cells expressed significant transcripts. The 5'-identity of macrophage IGF-I transcripts was established using an exon 2-derived IGF-I cDNA probe. All protected transcripts were foreshortened, indicating transcript initiation exclusively within exon 1, characteristic of extra-hepatic IGF-I mRNA. However, at the 3'-end, both IGF-I Ea (lacking exon 5) and IGF-I Eb (containing exon 5) mRNA transcripts were evident, with the Eb product being detected at levels similar to those present in hepatic cellular RNA. A large molecular size (26 kilodaltons) prepro-IGF-I peptide was also detected in macrophage cell lysates by Western blotting. Collectively, our observations show that: 1) among hemopoietic cells, myeloid rather than lymphoid cells are the major source of IGF-I; 2) macrophage IGF-I mRNA consists of class I Ea and Eb transcripts; 3) these transcripts are translated into protein; and 4) expression of IGF-I is directly associated with differentiation of bone marrow macrophages.