The combined action of the antitumor antibiotic bleomycin and chloroethylnitrosourea (CCNU) was studied in human lymphocytes in vitro. All the experiments were carried out with 20 micrograms/ml bleomycin for a given treatment time. By adding 0.7 and 3.5 micrograms/ml CCNU at late G1-S phase we have demonstrated a considerable increase in both percent of aberrant cells and production of dicentrics and rings (5-fold, p less than 0.001). At late S-G2 the combined treatment led to a significant enhancement of breaks per cell (p less than 0.0001) and cells with more than 12 aberrations. A possible explanation could be the known repair-inhibitory potential of CCNU, but its pure clastogenic action still has to be considered. The results presented here point out the need for seeking chemotherapeutic regimens with reduced concentrations of the drugs in combination.