An extensive exploration of the discriminating properties of olfactory receptors cells has been conducted in our laboratory through the statistical processing of single cell responses recorded in the frog's olfactory epithelium. Similarities between odorant stimulating properties could be demonstrated by comparing the complex response profiles of receptor cells, resulting in the concept of odorant groups which depicts consistent relationships found between some odorous compounds. The recording technique limits the number of odorants which can be tested and compared within the same series of experiments. Thus, based upon previous studies we have chosen a specific set of 20 odorants whose characteristics are known but, up to this time, have not been compared in the same experimental system. These odorants were: acetophenone, anisole, n-butanol, DL-camphor, cyclodecanone, 1,8-cineole, p-cymene, D-citronellol, n-heptanol, isoamyl acetate, isovaleric acid, D-limonene, methyl amylketone, L-menthol, phenol, thiophenol, pyridine, thymol, cyclohexanol, cyclohexanone. The pattern of similarities between these odorants, as delineated with the aid of correlation coefficient computation, factor analysis and non-hierarchical taxonomy, confirms the reality of several odorant groups previously suggested and describes their cross-relationships. The receptor mechanisms underlying these odorant groups are discussed. Because reliable prominent features of the olfactory stimulus space can be established on an objective basis, the findings are proposed as a reference for future studies on other aspects of olfactory discrimination.