Alloantigen-sensitized spleen cells suppressed MLC (mixed lymphocyte culture) reaction. In this paper the effect of levamisole (LMS) on MLC suppressor cells is reported. Spleen cells of C57BL mice were used as the responder for MLC, and spleen cells of BALB/c mice as the stimulator for MLC. Suppressor cells were prepared as follows: C57BL mice were injected with 2×10(7) BALB/c spleen cells into the hind foot pads. Four days later suppressor cell suspensions were obtained from the spleens. LMS-treated suppressor cells were prepared as follows: LMS was injected to C57BL mice once a day for 4 days after the foot pad injection of BALB/c spleen cells. After that, LMS-treated suppressor cells were obtained from the spleens. The dose of LMS was 1.25 mg/kg once a time. The stimulation index (SI) of one way MLC of C57BL mice to mitomycin C-treated BALB/c mice was 9.58, and mitomycin C-treated suppressor cells markedly suppressed the mitogenic response of C57BL responding cells to BALB/c stimulating cells, reducing the response by 88.9% . However, LMS-treated suppressor cells which were treated with mitomycin C markedly reduced this suppressive effect when these cells were added into MLC (responder: C57BL, stimulator: BALB/c) instead of suppressor cells. In contrast, when LMS was prior injected to C57BL mice, or before and after the foot pad injection of BALB/c spleen cells as other methods of LMS medication, LMS-treated spleen cells had no effect on MLC suppressor function. It seemed that LMS affected on MLC suppressor function only when it was given to mice with excessive suppressor cells.