The human insulin-like growth factor-I (IGF-I) gene codes for two transcripts, IGF-IA and IGF-IB mRNAs, formed by alternative splicing. In this study, the expression of these IGF-I mRNA transcripts was examined using human liver, hepatoma cells, macrophage-like cells and fibroblasts. The reverse transcription-polymerase chain reaction revealed that these cells contained both IGF-IA mRNA (representing exons I, II, III and V) and IGF-IB mRNA (representing exons I, II, III and IV). Interestingly, an RNase protection assay using 32P-labeled IGF-IA and IGF-IB exon-specific cRNA probes demonstrated that IGF-IA mRNA was 10-fold more abundant than IGF-IB mRNA in these cells. However, there was no difference in the stabilities of IGF-IA and IGF-IB mRNAs. These observations indicate that IGF-IA mRNA is more expressed than IGF-IB mRNA in these cells independent of their stabilities.