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1. A multienzyme complex containing at least DNA polymerase (EC 2.7.7.7), thymidine kinase (EC 2.7.1.21), dTMP kinase (EC 2.7.4.9) nucleoside diphosphokinase (EC 2.7.4.6) and thymidylate synthetase was separated from the corresponding free enzymes of DNA precursor synthesis by gel filtration of a gently lysed preparation of HPB-ALL cells (a human lymphoblastoid cell line). 2. The isolated complex incorporated the distal DNA precursors [ 3 H]thymidine or [ 3 H]dTMP into an added DNA template at rates comparable to those observed using the immediate precursor [ 3 H]dTTP. Measurement of the apparent overall concentrations of [ 3 H]dTTP produced during incorporation of [ 3 H]thymidine and of [ 3 H]dTMP were so low as to suggest that these precursors were channelled into DNA by the operation of a kinetically linked complex of precursor-synthesizing enzymes and of DNA polymerase. 3. The DNA polymerase inhibitor 1-β- d -arabinofuranosylcytosine triphosphate reduced incorporation of distal precursors into DNA. However [ 3 H]dTTP did not accumulate in the reaction mixture. This suggested that the DNA polymerase regulated the flow of substrates through the complex. 4. The results in this paper constitute direct evidence for the existence of multienzyme complexes of DNA synthesis in mammalian cells.