Growth of normal, nontransformed cells is regulated by the interplay between growth stimulating compounds and growth inhibiting cell-cell contacts. We have previously shown that the growth of normal diploid human fibroblasts is mainly regulated by a specific class of plasma membrane glycoproteins (R. J. Wieser and F. Oesch (1986) J. Cell Biol. 103, 361-367). Because it was found that immobilization of the glycoproteins involved in contact-dependent inhibition of growth is an essential step in the recovery of the biological activity of the glycoproteins, we developed a technique for a first characterization of the active compounds. After SDS-PAGE separation of plasma membrane glycoproteins, they were transferred onto nitrocellulose. The nitrocellulose was cut along the separation track into circles which fit into wells of a 96-well microtiter plate. Culturing human diploid fibroblasts on the nitrocellulose circles resulted in characteristic growth patterns, which were dependent upon the source and the treatment of the plasma membrane proteins which had been separated. Five major inhibitory fractions with apparent molecular masses of 300, 170, 90, 50, and 25 kDa have been identified in plasma membranes from confluent fibroblast cultures.