In the microenvironment of many solid tumors the pH is considerably lower (mean pH between 6.6 to 7.2) than the pH in normal tissue (pH 7.0-7.5). Therefore, the influence of acidic pH on the cytotoxic activity of lymphokine-activated killer cells (LAK cells) after different culture periods was tested. K-562 human erythroleukemia cells were selected as target cells. Cell killing was measured using a two-color flow cytometric method. At physiological pH of 7.4, LAK cell-mediated cytotoxicity ranged from 15 to 48% (E:T ratio = 50:1). The specific lysis of target cells was considerably reduced (up to 70% inhibition of specific lysis) under acidic conditions (pH 6.8, 6.3, 5.8). This effect was independent of donors, duration of the culture period, and the E:T ratio in the cytotoxic assay. As pH gradients surrounding tumor cells may reach values below pH 6.0 at the cell surface, the pH-dependence of LAK cell cytotoxicity could at least partially explain the inhibition of the natural immune response in solid tumors. Therapeutic immunological strategies concerning the enhancement of the natural immune response like LAK cell and IL-2 immunotherapy including IL-2 gene therapy may only be successful if a simultaneous inhibition of the acidification process and an elevation of tumor pH is achieved.