The activity and stability of beef liver catalase have been investigated in the presence of different ionic and zwitterionic surfactants. Ah cationic and zwitterionic surfactants used in this work have no effect on the initial activity of catalase, but several of them allow the enzyme to retain a high residual activity for longer periods of time than those observed in the absence of any additives. However, the interactions between surfactants and catalase appear to be very peculiar, and certain zwitterionic surfactants have been found to remarkably : slow down enzyme degradation, with the enzyme completely preserving its activity after several weeks at temperatures of up to 30 degrees C. This effect is probably due to an interaction between the surfactant and the intersubunit region of the protein; this interaction could stabilize the quaternary structure of the enzyme.