We have previously shown that K1 capsular polysaccharide antigen (K1CPS) of Klebsiella exhibits anti-tumor activities. In the present study, we examined the effect of K1CPS on cytotoxic effector cells. We found that K1CPS could activate many cytotoxic effector cells including alloreactive cytotoxic T cells and tumor-infiltrating lymphocytes (TILs). Moreover, K1CPS could increase the anti-tumor activity of lymphokine-activated killer (LAK) cells, both in vitro and in vivo. The i.p. injection of K1CPS in low dose could enhance the LAK cytotoxicity and the effect was further potentiated by coculture of LAK cells with K1CPS and low concentration of murine rIL-2 in vitro. The phenotypic characterization revealed that K1CPS might contribute to the increase in CD3+ LAK cell subpopulation by its in vivo priming effect. In addition, the K1CPS-treated LAK cells were able to inhibit the growth of WEHI-164 tumor cells in vivo in Winn-type inhibition assay. Subcutaneous (s.c.) and intraperitoneal (i.p.) adoptive infusion of LAK cells (splenocytes from K1CPS-treated WEHI-164-bearing mice cultured with K1CPS-plus-rIL-2) into WEHI-164 sarcoma-bearing mice could slightly cause regression in terms of tumor diameter, and more significantly in sarcoma weight.