The hormone defined serum free conditioned medium (SFCM) of human nasopharyngeal carcinoma epithelioid cell line (CNE 1) was assaied by both the [3H] thymidine incorporation test and the soft agar test. It was found that the SFCM can stimulate the growth of long-term serum free cultured CNE 1 cells in accordence with the fact that the growth rate of long-term serum free cultured CNE 1 cells was directly proportional to the plating density. Alternatively 5% SFCM can inhibit the growth of short-term serum free cultured CNE 1 cells by 51% in which the indicator cell may remained the responsive state of growing in the serum supplemented medium to the effector of interest. Furthermore, SFCM resulted in inhibition of anchorage-independent growth of CNE 1 cells and A431 cells. Also in soft agar test, SFCM can reduce the colony formation of NRK-49F cells in the presence of EGF or EGF plus TGF-beta. These finding suggested that CNE 1 can autocrine growth stimulating factor(s) and growth inhibiting factor(s) in the serum free medium, the latter can strongly reverse malignant phenotypies of CNE 1 and A431 cells in serum supplemented surrounding.