Lactobacillus acidophilus R-26 is unique among bacteria in its dependence upon four deoxynucleoside kinase activities for the synthesis of the deoxynucleoside triphosphate precursors of DNA. The products of these essential activities are kept in balance by combinations of end-product and substrate-level modulations. Three of the four activities appear to be organized into two overlapping pairs of associated activities, as evidenced by their kinetic interactions and their co-purification. The pairs observed are dCyd/dAdo and dGuo/dAdo phosphorylating species. The fundamental issues to be explored in this study are, 1) whether or not these paired activities are examples of multi-functional proteins, and 2) how a number of catalytic and regulatory sites are organized and interact on a relatively small protein molecule.