8-Chloroadenosine 3',5'-monophosphate (8-Cl-cAMP) is a potential new anticancer agent, but its mechanism of action is not clearly defined. In this work we have studied the effect of various heat inactivated and heat untreated human sera in the absence or in the presence of a nonspecific phosphodiesterase (PDE) inhibitor, IBMX, or of nucleoside transport inhibitor and cGMP-specific PDE inhibitor dipiridamole (DP), or of inosine-monophosphatedehydrogenase (IMPDH) inhibitor, tiazofurin, (T), on the antiproliferative 8-Cl-cAMP action towards two human malignant cell lines, K562 and HeLa cells, in vitro. Cell survival was determined 72 hrs after the agents action, using MTT assay. The results obtained, indicated the similar inhibitory effect of 8-Cl-cAMP on HeLa cell survival in the presence of four different heat untreated human sera (IC50 = 4-4.8 microM). Serum heat inactivation caused decrease in 8-Cl-cAMP antiproliferative action depending on the blood donor (IC50 = 23 microM, 15 microM, 19 microM, and 9 microM) and suggesting that some thermolabile ingredient(s) present in sera is involved, at least partially, in the induction or permittance of antiproliferative 8-Cl-cAMP action. K562 Cells were not as much resistant to 8-Cl-cAMP as HeLa cells, or mouse melanoma B16 cells; in the presence of heat untreated FBS, IC50 = 16 microM, while for B16 cells IC50 was 8 microM. Different human sera show different effect on 8-Cl-cAMP action on K562 cells: IC50 = 7.5 microM and 16.5 microM. In the presence of heat inactivated human sera 8-Cl-cAMP IC50 concentrations were higher, with relevant mutual differences. The effect of different sera on 8-Cl-cAMP action was only partly abrogated in the presence of a nonspecific PDE inhibitor, IBMX, suggesting that the serum PDE action is one of the various factors contributing to the induction of 8-Cl-cAMP antiproliferative action. Nucleoside transport inhibitor and cGMP-specific PDE inhibitor dipiridamole inhibited the antiproliferative 8-Cl-cAMP action to HeLa and K562 cells. Tiazofurin and 8-Cl-cAMP acted as antagonists on HeLa, but not on K562 cells.