Inhibition of the growth of human cancer cells by proteins secreted from 373-L1 cells was investigated in the present study. The growth of human cancer cells was inhibited by co-culture with 373-L1 cells under 10% FBS and DME, DME, GIT and serumless medium, respectively. The conditioned medium of cultured 373-L1 cells under serumless medium was concentrated 100-fold through an ultrafiltration cell with a 10,000 molecular weight cutoff at 4$^{circ}C$ under positive pressure using nitrogen(373-L1 EM). 373-L1 EM inhibited the growth of HeLa, Hep G 2, KHOS-Np, A43l and MCF-7 cells. 3T3-L1 EM was purified with FPLC, DEAE-ion exchange chromatography and phenyl-sepharose chromatography. The major protein of 373-L1 EM has a molecular weight of 66,000-68,000 in SDS-PAGE analysis. The results suggest that the inhibitory activity of 373-L1 EM appears to be due to some protein(m.w.66,000-68,000) secreted by 373-L1 cells.