Calorimetry has been used as a rapid method for the determination of bioactivity. Some previous studies have been concerned with its applicability to derive structure activity relationships (SARs), which can in turn guide the synthetic chemisty through the synthesis of new chemical entities with improved antimicrobial potencies. However, earlier studies do not point out the use and importance of calorimetry in deriving quantitative structure activity relationships (QSARs), a field where it is possible not only to Set information on SARs, but insight into modes of action can also be envisaged. In this paper we describe a QSAR based on biological microcalorimetry for a set of antimicrobial hydrazides acting against Saccharomyces cerivisiae. Results show that an extrathermodynamic relationship can be drawn based upon microcalorimetrically measured biopotencies and partitioning (log P-TA) using the same cell system.