Abstract The purpose of this study was to investigate the effects of cytokines and retinoic acid in human thyroid cancer cell growth. Cellular proliferation studies of the CGTH W-1 and SW 579 cell lines were performed with various cytokines and all-trans retinoic acid (RA). Cell number was determined by cell counting and incorporation of [ 3H]thymidine into DNA. Inhibitory effects of tumour necrosis factor α (TNF-α) were found in both cell lines. SW 579 was more sensitive to TNF-α. The SW 579 cell line revealed gradually decreased cell proliferation in [ 3H]thymidine incorporation studies as TNF-α concentration increased. In contrast, the CGTH W-1 cell line revealed prominent suppressive effects when the TNF-α concentrations increased over 1 ng/ml. An inhibitory effect of interleukin 1β (IL-1β) on CGTH W-1 cells was noted at the concentration of 1 ng/ml, however, IL-1β failed to demonstrate an inhibitory effect in SW 579 cells.