An agarose matrix containing mouse renal adenocarcinoma (RENCA) cells, with potential antineoplastic activity. The agarose-agarose macrobeads consist of two spherical agarose layers; the mouse RENCA cells are contained within the inner agarose layer. Upon placement into the abdominal cavity, the restricted mouse renal adenocarcinoma cells in the agarose macrobeads produce and release certain growth-retarding factors that inhibit the proliferation of the RENCA cells. Upon diffusion of these growth-slowing factors out of the agarose layers, these substances may inhibit cancer cell proliferation of proliferating tumors.