An autocrine factor in CM regulates C/EBPβ resistance to proteolysis. CM from 3-day-old cultures of P388D1(IL1) cells was fractionated by sequential ultrafiltration, with the flow-through fraction from the 30-kD cutoff membrane applied to the 10-kD cutoff filter. Fractions were applied to freshly fed cells, the cells were cultured for 8 hours, and nuclear extracts were prepared by the hypotonic lysis procedure. C/EBPβ binding activities were analyzed by EMSA.