The immunomodulating potential of the thymic protein prothymosin alpha1 (Pro alpha1) on the lymphocyte and monocyte directed antitumor reactions of melanoma and colorectal tumor patients in comparison with healthy controls were studied in vitro. On average, tumor patients showed lower NK- and LAK-cell activities than healthy controls, being associated with a lower adhesion capacity to tumor target cells. The NK-cell activity of the tumor patients was inversely related to the tumor stage. Pro alpha1 stimulated the impaired patients LAK-cell activity only at an early stage of disease. The Pro alpha1 effects were associated with an increased adhesion of lymphocytes to tumor target cells and an increased secretion of deficient IFN-gamma and IL-2 secretion. By flow cytometry, Pro alpha1 in combination with IL-2 increased the NK-cell marker CD56, CD16/56 and CD25 as well as CD18/11a adhesion molecule expression. Monocytes from tumor patients showed deranged tumoristatic activities compared with healthy controls. Pro alpha1 elevated the mean of the antitumor activity, when applied alone or in combination with rIFN-gamma. In the presence of IFN-gamma, Pro alpha1 stimulated the adhesion of monocytes to cultured tumor cells, mainly by increasing CD54 expression. Pro alpha1 stimulated alone or in combination with IFN-gamma the TNF-alpha and IL-1beta secretion by monocytes and decreased the high PGE2 and TGF-beta level, especially in the patients groups. Perspectively, this may provide the basis for applying Pro alpha1 in selected antitumor immunotherapy protocols.