This study was designed to determine the effect of serial temperature on cell proliferation in the culture of epithelial cells obtained from oral mucosa of the adult rabbit and human reonate.Oral epithelial cells were collected by treatment with EDTA and trypsin. Swiss 3T3 cells which had been previously irradiated by 60Co (6, 000 rad) were used as a nutrient layer in some culture.A “primaria dish” was used in other cultures without nutrient, in order to determine the possible direct action of growth stimulating factors such as EGF, cholera toxin and retinoic acid. A modified MCDB 152 medium containing insulin, hydrocortisone and transferin was used. A new gradient temperature incubator able to simultaneously set different temperatures was used.The medium was changed every 3 days, and on the 8 th day when the maximum number of colonies was reached, the cultured materials were fixed and stained with May-Grtinwald's solution and Giemsa's solution respectively. Numbers of colonies of 8 or more cells were counted on the bottom surface of the dishes with 24 wells through an inverted microscope. The mean and standard deviation were calculated for every set culture consisting of 3 or 6 wells in the same condition. The statistical difference between the culture sets was obtained by the t-test.Results of this study show, that:1. Although a small fluctuation of between 0.2 to 0.4°C in average temperature was measured resulting from room temperature changes, the water cooling incubator system was found to be reliable, so that relatively stable temperatures were obtained at every temperature setting.2. When oral epithelial cells from the adult rabbit were cultured at the low-temperature level of 33. 4-34. 0°C, the number of colonies was greater than at the high-temperature level of 36. 0-37. 0°C, regardless of the use of a feeder layer.3. Colony formation was stimulated by the addition of EGF and cholera toxin. However, colony formation was still greater at the low-temperature level. It was therefore obvious that temperature acted on colony formation independently from stimulation.4. The addition of retinoic acid increased colony numbers regardless of the use of the feeder layer at a temperature range of 32. 0 to 35. 0°C. Numbers decreased at the low tem-perature of 31. 8 °C and the high level of 36. 1°C. Adenocyte-like colonies, frequently observed when retinoic acid was added, showed a similar thermal response.5. The number of colonies of fibroblast-like cells remained small when cultured at the low temperature of 33. 0 C. However, numbers increased as the temperature rose. Therefore, at least as far as rabbits were concerned, a distinct difference was found in the optimal temperature for the culture of epithelial and fibroblast-like cells.6. For immature-type colony formation of human new-born epithelial cells, the low temperature level of 34. 4 QC was most effective. However, blast-cell type colonies multiplied in a thermal-dependent fashion and the high-temperature of 36. 5 °C was optimal.In addition, number of cells per colony reached maximum at the high temperature level of between 35. 6 and 36. 8°C.7. Previous observations of epithelial cell-type growth were confirmed; it become obvious that the optimal temperature range for proliferation differed for the two cell types.Furthermore, it appears that the blast-cell type colony originated from either epithelial stem or progenitor cells and that the immature-type colony cells were markedly similar to cells of the suprabasal region of the epithelium.