The heat production of normal and transformed human epidermal keratinocytes precultured in Petriperm tissue culture dishes was measured calorimetrically. For this purpose, the membrane at the bottom of the culture dish was cut out aseptically and put into a microcalorimeter vessel with the cell layer inwards. A continuous heat output of (83 +/- 12) pW/cell was measured for normal keratinocytes from a confluent primary culture. A value of (134 +/- 35) pW/cell was obtained when the transformed keratinocyte line SV-K14 was used. The method described in this paper is simple, leads to reproducible results, and can be easily adapted to the calorimetric study of other mammalian cells in vitro.