Deoxyguanosine kinase (ATP: deoxyguanosine 5'-phosphotransferase) activity has been identified in neonatal mouse skin tissue. This activity which has a molecular weight of 44 000 was shown to be highly specific for deoxyguanosine as a substrate. Unique to deoxynucleoside kinases was the observation that this enzyme possessed a pH optimum of 5.2. In dilute solutions catalytic activity is lost, however the enzymatic activity can be stabilized by the addition of 20 micrometer MgATP or ATP. Kinetic analysis gave an apparent Km for deoxyguanosine of 7 micrometer. Double-reciprocal plots of activity vs. MgATP concentration produced a broken line with the break occurring at 0.5 mM MgATP. Below this concentration an apparent Km for MgATP of 23 micrometer was measured; above 0.5 mM MgATP an apparent Km of 265 micrometer was calculated. Mouse skin deoxyguanosine kinase was strongly inhibited by dGTP, dGDP and UDp. dGTP was a competitive inhibitor of deoxyguanisine with an apparent Ki of 1.9 micrometer. UPD (K1,app = 3 micrometer), dGDP = 0.7 micrometer) (Ki,app = 0.07 micrometer] were competitive inhibitors of MgATP and dGTP ki,app = 0.07 micrometer) were competitive inhibitors of MGATP when the concentration of MgATRP was greater than 0.5 mM.