Interactions of novobiocin (NB) and/or nalidixic acid (NA) with some cytotoxic agents--UV light (UV), X-rays, methylmethanesulfonate (MMS), bleomycin (BM), adriamycin (AM), cis-diaminedichloroplatinum(II) (CDDP), mitomycin C (MIT), ethidiumbromide (EB), and suramin (SA)--have been investigated in thymic (T) and splenic (S) cells of the rat in vitro by measuring semiconservative (SDS) and unscheduled (UDS) DNA synthesis as well as sedimentation and viscosity of nucleoids. Combining NB (900 micrograms/ml) and/or NA (1800-3600 micrograms/ml) with UV, MMS, BM, AM, MIT, and SA resulted in additive effects on SDS and UDS. Synergistic actions were observed in T- and S-cells simultaneously treated by NB and CDDP, whereas the effects of NB could be antagonized to some extent by EB and vice versa. In X-irradiated (greater than or equal to 28 Gy) cells pretreated by NB (NA), UDS was diminished (T-cells) or enhanced (S-cells). The results are consistent with the following postulates: 1 degree in S-cells, DNA is much more supercoiled than in T-cells but in the opposite sense (positive superhelicity). 2 degrees DNA supercoiling (DNA compactness) is influenced therefore by DNA-topoisomerase inhibitors and intercalating agents in a highly agent- and cell-specific manner.