The effect of arabinosyl cytosine (ara-C) was studied on the uptake, phosphorylation and incorporation of 3H-thymidine in human tonsillar lymphocyte cultures is described along with its effect on the level of DNA polymerase and thymidine Kinase activities induced by phytohaemagglutinin (PHA). Freshly isolated tonsillar lymphocytes are stimulated cells with a remarkably high activity of DNA polymerase alpha and thymidine kinase. During in vitro culture, these stimulated cells are transformed to the resting state with low DNA polymerase and thymidine kinase activity. However, a new DNA synthesising cycle can be induced by PHA with maximum at 48 h. 10(-6) M ara-C inhibited the incorporation of 3H-thymidine by 90-95%. This inhibition may be reversed by rinsing the cells. The inhibition of the transport of 3H-thymidine seems to be only a consequence of the inhibitory effect of ara-C on the DNA polymerisation reaction, because at 10 degrees C, where DNA synthesis was arrested, ara-C does not influence the uptake and the phosphorylation of 3H-thymidine. Ara-C (10(-6) M) abolished also the PHA induced elevation of DNA polymerase alpha and thymidine kinase activities without influencing protein synthesis of the cell. This supports a coordinated regulation mechanism between DNA synthesis and the synthesis of enzymes involved in DNA replication.