The biodegradation of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) by Pseudomonas cepacia was assessed by microcalorimetry in a liquid medium and in sterilized soil at 25 degrees C under aerobic conditions. It was found that thermograms of the rate of heat evolved versus time (dQ/dt versus t) can be used as a diagnostic tool to identify the time t(1) required for the primary biodegradation of 2,4-D and the time t(f) required for the completion of the biodegradation activity in a liquid medium as well as in soil. Microcalorimetry can also be used as an analytical tool to monitor the progress of 2,4-D consumption during the biodegradation process in a liquid medium and to measure the importance of the soil sorption/desorption of intermediate metabolites. A new concept called "bioeffort" was defined as the product of the biodegradation time (t) and the biomass concentration (X) at that time. This concept was used to predict either the biomass concentration required or the duration of the primary biodegradation of 2,4-D in soil from the data obtained from a liquid medium.