Abstract: Apparatus and method are described for heating and/or cooling a plurality of objects simultaneously at different preselected temperatures, wherein a predetermined temperature difference is established between two spaced points on a heat-conductive member to produce a constant temperature gradient between the points; and then a plurality of objects are supported in heat exchange relationship with respect to predetermined locations of the temperature gradient according to the preselected temperatures to which the respective objects are to be heated or cooled. The apparatus further includes temperature control means for maintaining the two spaced points of the heat conductive member at the predetermined temperature difference during the heating or cooling of the objects.