The interactions of the two antitumor protein antibiotics, neocarzinostatin (NCS) and bleomycin (BLM), were studied on subcellular and cellular levels. BLM and NCS were found to remove thymine from double-stranded DNA. Combination experiments using BLM and NCS together in an assay with isolated DNA revealed an additive effect in splitting. Under limiting concentration conditions, BLM and NCS induce alkali-labile sites in DNA without a subsequent cleavage of the chain. After transfer of BLM- or NCS-treated DNA into an alkaline solution, strand scissions occur. Combination of BLM and NCS results in an additive DNA-cleaving effect, which indicates that the splitting reactions initiated by BLM or NCS are not influenced if the two antibiotics are applied in combination. The DNA polymerase beta is inhibited by BLM (at higher concentrations) and by NCS in a competitive way with respect to DNA. The inhibition constant of BLM and NCS in a combination experiment was found to be the result of the sum of the inhibition constants of BLM and NCS. Using L5178Y mouse lymphoma cells, it was found that cells incubated with both BLM and NCS show "unbalanced growth." The dose-response curves from BLM and NCS have identical slopes; they are characteristic for compounds which selectively inhibit DNA synthesis. By use of isobolograms, it could also be clearly shown that BLM and NCS interact additively.