Leukocytes synthesize a variety of hormones that were once thought to be unique products of endocrine tissues. Understanding the regulation of leukocyte-derived hormone synthesis requires an accurate means for measuring steady-state expression of specific mRNA transcripts. Here we describe a competitive reverse transcriptase-polymerase chain reaction (RT-PCR) technique to accurately quantitate macrophage-derived insulin-like growth factor-I (IGF-I) mRNA, and demonstrate the utility of this approach for measuring expression of leukocyte-derived hormone transcripts. A riboprobe was constructed to generate approximately 1 kb of synthetic competitor IGF-I RNA (exons 1 and 3-6) that differed from cellular IGF-I RNA by insertion of 122 bp of β-actin RNA. One set of oligonucleotide primers could thus be used to simultaneously reverse transcribe and amplify both 144 bp of cellular (exons 3 and 4) and 266 bp of competitor IGF-I RNA. Densitometric scanning of the PAGE-separated PCR products revealed that the ratio of competitor to cellular amplified DNA bore a linear relationship (r2≥. 0.98) to the amount of competitor RNA for both rat liver and splenocytes. However, rat liver contained 104 x 106 IGF-I molecules per microgram of total cellular RNA compared to only 2 x 106 IGF-I molecules for splenocytes. Competitive RT-PCR using as little as 0.5 μg cellular RNA, coupled with [α-32P]dCTP incorporation in the amplification step and phosphorimager analysis of the resulting gel, was then used to show that colony-stimulating factor-1 (CSF-1) induces an increase from 2 x 106 to 110 x 106 IGF-I molecules per microgram RNA as bone-marrow-derived progenitors differentiate into macrophages in vitro. Quantitation of these same transcripts by RNase protection assays revealed that CSF-1 induced a similar 60-fold increase in IGF-I mRNA, but 15 μg target RNA was used for this assay. These data confirm that IGF-I transcripts are coordinately expressed during macrophage differentiation induced by CSF-1. They also show the utility and broad application of using synthetic, competitor RNA derived from an expression vector as an internal standard in RT-PCR for quantitating steady-state transcripts for leukocyte-derived hormones.