Ribonuclease inhibitor (RNH) is a protein that binds tightly to ribonucleases in cells and may be essential in the control of mRNA degradation and gene expression. The human RNH gene has been regionally localized to chromosome band 11p15 by in situ hybridization. A human placental cDNA was used to construct a 600-bp probe, which was then radiolabeled with tritium for in situ hybridization to human metaphase chromosomes. Localization of the RNH gene to 11p15, and possibly to 11p15.5, adds to a large number of genes assigned to this band, including 10 structural genes. This chromosomal region also represents an evolutionarily conserved syntenic group in the owl monkey, mouse, rat, and cow. Thus, regional assignment of RNH could facilitate the understanding of this gene and its association with ribonucleases, and perhaps extend a conserved syntenic region in mammalian genomes.