A statistical method for radiation hybrid (RH) mapping of a small number of genes detected in clones by means of isozyme analysis is suggested. This method allows analytical solution of the problem of gene ordering and qualitative comparison of relative distances between the genes that are being mapped. The possibilities offered by the suggested method are illustrated using earlier data on the distribution of four isozyme markers in human and vole (Microtus rossiaemeridionalis) clones. These markers are hypoxanthine phosphoribosyl transferase (HPRT), PGK, G6PD, and GALA.