The aim of this study was to evaluate expression of receptor for IL-1 on tumor-derived cells. The in vivo acquisition of an expression of a receptor for Fc ç immunoglobulin on polyoma virus transformed cells has been established by us. We investigated whether a receptor for the IL-1 cytokine, like that for Fc gamma immunoglobulin, could also contribute to the heterogeneity of tumor cell population, as well as to its tumorigenic phenotype. Various clones of polyoma virus transformed 3T3 cells were passaged once in syngeneic mice and resulting tumors explanted and recultured. The expression of receptor for IL-1 was tested on in vitro maintained clones (designated C for culture) and on tumor derived clones (designated CTC - culture-tumor-culture). Expression was determined using a 125I radiolabeled ligand and confirmed by flow cytometry with anti-mouse IL-1 receptor (IL-1R) antibodies. Some CTC clones expressed a higher level of IL-1 receptor than others. A positive correlation between the level of IL-1R and a metastatic phenotype was established with some tumor derived cells. A high IL-1R expressing tumor cell population, sorted by flow cytometry, was considerably more metastatic than the sorted low IL-1 expressing cells. IL-1R expression by tumor derived cells may, contribute to the metastatic phenotype of a tumor cell population.