A number of temperature-sensitive cdc- mutants of Schizosaccharomyces pombe that are affected in DNA replication, were screened for the absence of deoxynucleoside triphosphate(s) when blocked at their restrictive temperature. The preliminary screening simply involved analysis of perchloric acid-soluble cell extracts by two-dimensional thin-layer chromatography on poly(ethyleneimine)-impregnated cellulose. One mutant strain, cdc 22-M45, was found which apparently lacked dTTP. Pulse-labelling of intracellular nucleotides revealed that not only did dTTP become depleted, but that dTDP accumulated when this mutant was blocked by a temperature shift-up, indicating a defective nucleoside diphosphokinase. Nucleoside diphosphokinase from cdc 22-M45 was less active than that from wild-type strain 972 when assayed at high temperatures. The nucleoside diphosphokinase of the mutant also has an altered Km for dTDP at both permissive (25 degrees C), and at the restrictive (36.8 degrees C) temperatures. At the restrictive temperature the Km for dTDP of the mutant enzyme is more than 11-times greater than that of the wild type. Characterisation of the biochemical basis of the defect in this cdc- mutant has shown that in S. pombe, despite its having an apparently complex system of genetic control over progression through S-phase, one factor at least is merely availability of a nucleoside triphosphate precursor to DNA synthesis.