Bacterial adhesion and coaggregation are involved in the development of oral biofilms, called dental plaque. Although various techniques have already been used to study different aspects of these bacterial interactions, microcalorimetry has not yet been applied. This paper describes how isothermal reaction calorimetry can be employed to determine the enthalpy of coaggregation between two oral bacterial pairs. For most biological processes, the enthalpy tends to reach a minimum value, reflecting the most stable state, which is directly related to the heat content of the system. The calorimeter consists of four measuring units where reaction ampoules are filled with 1.5 ml of an Actinomyces naeslundii 147 suspension, while reference ampoules are filled with buffer only. After equilibration at 25 °C, 80 µl of a streptococcal suspension was titrated into the reaction ampoules. To study possible saturation of the binding sites on the actinomyces surface, three consecutive injections with streptococcal suspensions were done. Following each injection, a 20-µl aliquot was taken from the ampoule kept outside the calorimeter and the number of free (Sf) and bound (Sb) streptococci was determined microscopically. Experiments were carried out with a coaggregating streptococcal strain (Streptococcus oralis J22) and a non-coaggregating strain (Streptococcus sanguis PK1889), serving as a control. The coaggregation enthalpy was exothermic, that is, heat was released in the reaction ampoule upon coaggregation and the heat released by the coaggregating pair minus the heat released by the non-coaggregating pair yielded a coaggregation enthalpy of -0.015×10-6 mJ/bound streptococcus for the first injection. Upon consecutive injections, the coaggregation enthalpy decreased to -0.0004×10-6 mJ/bound streptococcus. Comparison with enthalpy changes reported for lectin–carbohydrate binding suggests that a huge number of binding sites are involved in the formation of one bacterial coaggregate.