Progressive tumor growth is associated with a state of immunosuppression. One mode of immunosuppression is thought to be mediated by immunosuppressive factors of tumour origin. We have investigated in vitro the possibility that suramin could be used to blockade tumor derived suppressor factors and enhance the effectiveness of rhIL-2 therapy. These data show that factors derived from cultures of the colorectal carcinoma cell line LoVo suppressed the percentage of cells expressing the natural killer cell antigen CD56 in 92% of individuals and the cytotoxic T cell antigen CD8 in 77% of individuals tested from a panel of 13 normal healthy volunteers. Suramin at 200 micrograms/ml restored the percentage of cells expressing CD56 to levels higher than the control cultures and reduced the suppression in those expressing CD8 to non-significant levels. LoVo produced factors also suppressed the expression of the activation associated antigens CD25, CD71 and HLA-Dr with suramin restoring CD25 expression but not CD71 or HLA-Dr. Functional studies using 51Cr-release assays showed that LoVo produced factors could suppress cytotoxicity in 46% of individuals tested, and of these a reduction in suppression by suramin was demonstrated in 50% of individuals against Daudi target cells and 33% against K562 target cells.