We have synthesized and evaluated a series of hybrids, denoted 22--27, for in vitro cytotoxic activity against a variety of cancer cell lines. These hybrids represent a molecular combination of polypyrrole minor groove binders structurally related to the natural antitumor agent distamycin A and two pyrazole analogues of the left-hand segment called cyclopropylpyrroloindole (CPI) of the potent antitumor antibiotic (+)-CC-1065. These novel water-soluble hybrids have been designed to enhance the minor groove binding ability of alkylating units 20 and 21, which should increase their clinical appeal by overcoming the administration problems of (+)-CC-1065 derivatives. The DNA alkylating and cytotoxic activities against several tumor cell lines are reported and discussed in terms of their structural differences in relation to both the number of N-methyl pyrrole rings and the type of the alkylating unit tethered to the oligopeptidic frame. It may be noted that, in general, and especially for 22--24, the cytotoxicity of the hybrids was much greater than that of the alkylating units alone. In only one case, compound 27, did the hybrid have cytotoxic activity comparable to that of the alkylating unit alone against FM3A/0 cells. The broadest spectrum of activity and greatest potency was shown by the hybrid 24, in which the alkylating unit 20 and the deformyl distamycin A are tethered by 1-methyl 2,5-dicarbonyl pyrazole, with IC(50) values for the different tumor cell lines ranging from 7 to 71 nM. For compounds 22--24, the increase of the length of the pseudopeptidic moiety from one to three N-methylpyrrole residues led to an increased cytotoxicity. Among the hybrids tested for their inhibitory effects on the proliferation of murine L1210 leukemia cell line, compound 24 proved to be the most active (IC(50) = 7.4 nM), and in the sequencing gel experiments, it showed the strongest and most highly sequence-specific DNA alkylation activity. For compounds 22-24, the sequence specificity of DNA alkylation appears to be affected by the modification of the number of pyrrole rings, and the correlation between cytotoxicity and alkylation pattern suggests that 24 exerts its cytotoxicity through DNA sequence-specific alkylation of the third adenine located in the sequence 5'-ACAAAAATCG-3'. The two other hybrids 22 and 23 were slightly less active for tumor cell proliferation, with IC(50) values of 58 and 19 nM, respectively. With only one exception, none of the compounds was endowed with antiviral activity at subtoxic concentrations. Compound 24 inhibited the effect of vaccinia virus at a concentration that was significantly lower than its minimum cytotoxic concentration for the E(6)SM host cells. These compounds gave distinct patterns of alkylation in AT-rich sequences, indicating that minor structural changes produced marked alterations in sequence selectivity.