We have studied the in vitro effect of IFN-alpha and bcr-abl antisense oligodeoxynucleotides (As ODN) alone and in combination with the aim of enhancing the antileukemic activity of the two single agents and evaluating whether the two agents in combination might restore the adherence capacity of chronic myeloid leukemia (CML) progenitors to preformed stroma. We have also correlated the increased adhesion found after in vitro treatment with the expression of adhesion molecules on leukemic progenitors. Incubation of the BV173 cell line with escalating doses of IFN-alpha (100-10000 U/ml) showed a colony growth inhibition between 10 and 30%. IFN-alpha and junction-specific As ODN in combination showed a greater antiproliferative effect compared to that observed with the two agents used alone. In particular, As ODN at a concentration of 40 microg/ml in combination with IFN-alpha at 100 and 1000 U/ml showed a greater inhibitory effect compared to that obtained with IFN-alpha only. Addition of As ODN to IFN-alpha at 10000 U/ml did not result in a greater BV173 inhibition. In a further set of experiments, primary cells from 16 CML patients at diagnosis were incubated with 40 microg/ml of J-spec As ODN, several control ODNs and IFN-alpha at 1000 U/ml alone and in combination. A significantly greater elimination of CML progenitors was found after treatment with the combination of IFN-alpha and J-spec As ODN, compared to any other treatment group, confirmed also by a more marked effect on p210 expression. The deficient adhesion of CML progenitors on human preformed stroma was restored at levels similar to that of normal bone marrow cells after treatment with IFN-alpha and/or J-spec As ODN, while the phenotypic analysis showed that the combined treatment increased significantly the expression of CD49b and CD62L on CML CD34+ cells. However, when the expression of adhesion molecules was blocked with specific monoclonal antibodies, only CD49d (expressed on more than 90% of CML CD34+ cells) appeared to influence the functional activity of adhesion molecules. In conclusion, IFN-alpha and bcr-abl As ODN in combination exert a marked in vitro antileukemic activity and could be a useful approach for in vitro purging of CML cells prior to autologous transplantation.