We describe the effect strongly influencing the biological activity of some camptothecin (CPT) drugs, the inhibitors of DNA topoisomerase I (topo I), namely, the formation of J-type aggregates in an aqueous buffer solution. These aggregates were built up under certain dilution conditions of the stock DMSO solutions of 20-S-camptothecin (20(S)CPT), 10,11-methylenedioxy-CPT (10,11-CPT) and 7-ethyl-10-hydroxy-CPT (SN38). The aggregates were found to be stereospecific, not being detectable for the 20(R)-stereoisomer of CPT. They were formed by the stacking interaction between quinoline rings of CPT chromophores with the inverse position of the nitrogen atoms. The aggregates were stable at acidic and neutral pHs, but dissociated at basic pHs. Self-aggregation prevented hydrolysis of the lactone ring at neutral pHs, thus preserving the drugs in a biologically active form. Addition of BSA did not induce either disaggregation or hydrolysis of the lactone ring, whereas the monomeric form of the drugs was shown to undergo rapid conversion to an inactive carboxylate form in the presence of human serum albumin [5]. The drugs did not form the aggregates in the presence of topo I. Moreover, rapid dissociation of the aggregates was observed if a self-aggregated drug solution was added to topo I alone or to the DNA-topo I cleavage assay. Neither DNA alone nor oligonucleotides derived from the sequences of the CPT-enhanced or topo I-induced cleavage sites in SV40 plasmid DNA induces changes in the aggregation state of the drugs. These observations are indicative of interaction between the aggregates and topo I. The aggregates were found to penetrate within the cells with much higher efficiency than a monomeric form of the drugs. Cellular uptake of aggregated and nonaggregated species correlated well with cytotoxic effects produced by the drug. In this manner, CPT's self-aggregation should be regarded as a favourable phenomenon producing species with a more stable biologically active structure of the lactone ring and exhibiting enhanced cellular uptake levels relative to the monomeric forms of medications.