The efficacy of guanidine hydrochloride (GuHCl) addition on the recovery and catalytic activity of Rhizopus delemar lipase was investigated in an AOT microemulsion system, The activity yield of lipase was markedly increased by the addition of GuHCl, and was maximum at a GuHCl concentration of ca 0.08 mol dm(-3). Addition of GuHCl was similarly effective in enhancing enzyme recovery after the hydrolysis of triolein. A kinetic study of the hydrolysis reaction was made as a function of GuHCl concentration. The reaction followed a non-competitive inhibition mechanism. The K-m remained constant at GuHCl concentrations from 0 to 0.12 mol dm(-3), while the maximum reaction rate at a GuHCl concentration of 0.08 mol dm(-3) was only 30% lower than that in a GuHCl-free system.