Studies on the activity of the enzyme horseradish peroxidase (HRP) have been carried out in micellar as well as reverse-micellar media. The activity of the enzyme was studied in the presence of different classes of surfactants - ionic as well as non-ionic. In aqueous media, the activity of the enzyme varied depending on whether the concentration of the surfactant used was above or below the critical micellar concentration (CMC). The enzyme was also studied in reverse-micellar systems. HRP was introduced into the reverse micellar phase by the injection method and its activity within the reverse micelles was determined. The effect of water to surfactant ratio (W-o) on activity within reverse micelles was studied, and an almost two-fold increase in activity was seen when the enzyme was encapsulated within reverse micelles of aqueous phase fractional hold-up (phi) of 0.0072 (v/v) consisting of sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) in isooctane at a W-o of 20. The activity of HRP was measured over a wide range of AOT concentrations having different W-o values. Back-extraction of HRP from these reverse micelles was carried out at varying ionic strengths of phosphate buffer. Back extraction was found to be highest at pH 7.0 in 40molm(-3) phosphate buffer and 100molm(-3) sodium chloride.