A material for suppressing proliferation of cancer cells is produced by entrapping cancer cells in a selectively-permeable structure such as a bead, and culturing the entrapped cells in a culture medium. Entrapment restricts growth of the cancel cells during culturing and causes the cells to produce in the culture medium a material having a molecular weight of at least about 30 kd that suppresses proliferation of cancer cells. The material is separated from the culture medium by filtering the medium through a filter that separates material having a molecular weight of at least about 30 kd from material having a molecular weight of less than 30 kd. The structure that entraps the cells may contain 10,000 to 500,000 cells.