A study of some characteristics of deoxynucleotide kinase activities in mouse-liver homogenates was made. TMP kinase and deAMP kinase, but not deCMP kinase were affected by the kind of homogenizing medium used. TMP kinase was unique in having a relatively high affinity for its substrate. At low substrate concentration the deoxynucleotide kinase activities of mouse-liver homogenate were about the same, except for deAMP kinase activity which was ten times higher. The enzyme level of TMP kinase in mouse and rat liver was low compared with the levels of the kinases of deCMP, deGMP and deAMP. The incorporation of TMP and deGMP into a phosphatase-resistant form indicative of di- and triphosphate synthesis, was characterized by interference from degradative activities which tended to prevent accumulation. The incorporation of deCMP and deAMP, however, did not show these characteristics. The results are discussed in relation to possible implications in the control of DNA synthesis in mouse liver.