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SUMMABY. An attempt to increase the heat resistance of two strains of Microbmtdum lacticum waa made by exposing cells dispersed on an agar surfece to 12" for two different times. Eighteen succeeeive subcultures from survivors gave no increase in the proportion of heat resistant cells. There waa no change in the ratio of the percentages of cells surviving the different treatments, an indication that there wm no change in the overall heat resistanae. With both strains the changhg proportions of colonies developing on the 3rd and 6th day of incubation demonetrated the damaging effect of increesing the time of exposure at 72" but did not indicate any increase in heat resistance. The theoretical implications of these hdinga are discuseed.

Summary: The new technique depends upon spreading bacterial cells over the surface of an agar medium in a Petri dish. The cells are heated, cooled, grown and counted in situ. The plates are preheated on the surface of a water bath and held for the required time on the surface of a second bath at the desired temperature. They are cooled by cold water jetted on to the base. The experimental control of factors affecting the accuracy of the method are discussed and applications suggested.