Five adenosine analogues resistant to adenosine deaminase were analyzed for their effects on the proliferation on cultured Vero cells. Cells were exposed for 48 hr to various concentrations of 1-deazaadenosine (c1Ado), 3-deazaadenosine (c3Ado), 7-deazaadenosine (c7Ado), 1,3-dideazaadenosine (c1,3Ado) or 1,7-dideazaadenosine (c1,7Ado) and then counted with a hemocytometer. Results indicate that 1) whereas c7Ado produced a marked decrease in cell growth, c3Ado was toxic only at the highest concentration tested (30 microM), 2) c1Ado and c1,7Ado did not affect cell replication and 3) c1,3Ado produced a significant stimulation of cell proliferation. Further studies have suggested that none of the compounds tested is incorporated in the DNA.