Using a severely controlled temperature gradient incubator, the difference in temperature sensitivity between normal and malignant human cells in vitro was studied. In this study, the inhibitory effect of cell proliferation and the degenerative changes on cell morphology of normal human diploid (HAIN-44 and 55) and endometrial adenocarcinoma cells (SNG-M) for about two weeks at various temperatures (32-43 degree C) were investigated. When the temperature was lower than 34 degree C, the growth of SNG-M, similar to that of the HAIN, was markedly suppressed, however, it was reversible. The cytoplasmic degenerative changes were also found in these cells. By contrast, SNG-M was more heat sensitive at and over 39.5 degree C than the HAIN, which continued to grow until 42 degree C. In other words, permissive temperatures to cell growth of SNG-M were approximately 3 degree C lower than those of the HAIN. Especially at 41 degree C and/or higher, SNG-M could not grow further and drastically degenerated within few days. Under such hyperthermic conditions, the cells were irreversibly damaged with fetal nuclear degenerative changes. No specific distribution pattern of chromosomal number was found at any temperature in the cases studied. It was clarified that there was a significant difference in heat sensitivity between the HAIN and SNG-M.