The pesticide picloram (4-amino-3,5,6-trichloropicolinic acid) was anchored onto silica gel to yield a new surface. Isothermal microcalorimetry was applied to study the toxic effects caused to microbial activity of a typical Brazilian agricultural soil by application of free and immobilized picloram. The activity of the microorganisms in 1.50 g of soil sample was stimulated by addition of 6.0 mg of glucose plus 6.0 mg of ammonium sulfate under 34.8% controlled humidity at 298.15+/-0.02 K. The activity was recorded through power-time curves for increasing amounts of the active principle, varying from zero to 10.00 mug g(-1). The increasing amounts of picloram, either free or immobilized, caused a decrease of the original thermal effect. The calorimetric data showed that the anchored pesticide presented a much lower toxic effect than the free picloram on the microbial activity.3765686690