We have examined properties of IMPD activity in soluble extracts from immortalized murine epithelial and fibroblastic cells. The absence of significant xanthine oxidase activity in these extracts allowed the use of a spectrophotometric assay to study the enzyme activity. The observed enzymatic activity had subcellular localization and kinetic properties similar to those of previously described mammalian IMPD from other sources. Analysis of IMPD activity in extracts from cells in different states of growth related to serum concentration gave a surprising result. Extracts from exponentially growing cells exhibited a level of IMPD activity similar to that of extracts from quiescent cells arrested by serum-deprivation. In previous studies, the cellular variable designated to account for changes in IMPD activity was proliferative rate. Our findings suggest that either proliferative rate is not the functionally significant variable related to IMPD regulation or that there are other factors that can supersede it in certain contexts. Given the role of the enzyme in regulating the synthesis of guanine nucleotides, which are key regulatory molecules for many cellular processes, this may indeed be the case. Using immortalized cell lines growth-arrested by serum deprivation, we have experimentally isolated the enzyme activity from the previously assigned variable of growth rate. Based on our findings we propose that regulation of IMPD activity is more appropriately related to proliferative capacity as opposed to proliferative rate.