Studies of interactions between drugs and biological material form a broad application area for isothermal microcalorimetry. Characterization of binding of drugs and related compounds to well-defined biological substances by use of titration microcalorimetry is a very important area. This technique has now been taken into use in practical work ("rational drug design") by several laboratories in the pharmaceutical industry. More fundamental studies of solute-solvent interactions for drugs and for simple model systems should be encouraged. Microcalorimetric techniques suitable for investigations of the effect of drugs on microorganisms and animal cellular systems have been much developed during recent years. However, few scientific groups are presently active in this field and it seems as if the technique has not yet been adopted by R&D laboratories in the pharmaceutical industry or in applied clinical work.