In vitro spontaneous proliferation is the immunological hallmark of peripheral blood mononuclear cells (PBMC) from HTLV-1-infected individuals. Quinoline compounds down regulate in vitro cell proliferation of HTLV-1 transformed cell lines. In the present study we assessed the capacity of quinolines to inhibit spontaneous cell proliferation of PBMC from HTLV-1-infected individuals. Twenty-two quinolines were evaluated. Toxicity was first assessed on PBMC from healthy donors by using both the Trypan blue technique and Tetrazolium Salt (XTT) method and then the antiproliferative effect was measured by a classic lymphoproliferative assay on PBMC from three HTLV-1-infected individuals, in the presence of decreasing concentrations of quinolines (from 100microM to 0.8microM), after 5 days of culture. We found that 14 out of 22 compounds were non-toxic to PBMC from uninfected individuals at 100, 50 and 10microM. Four compounds presented a capacity to inhibit more than 80% of the spontaneous proliferation: 7 at 25microM and 10, 20 and 23 at 100microM. Our results indicate that some quinolines block spontaneous proliferation of PBMC from HTLV-1-infected individuals.