Normal erythrocyte catalase, the enzyme present in the blood of Swiss acatalasemic heterozygotes, and their hybrid produced in vitro, were studied after crosslinking with bifunctional reagents. On theoretical grounds [cf. Hajdu, J., Bartha, F. & Friedrich, P. (1976) Eur. J. Biochem. 68, 373--383] it is inferred from the dodecylsulphate gel electrophoretic patterns obtained after treating catalase with diimidates of various chain lengths that the enzyme is an isologous tetramer (D2 symmetry). The minimal distances between crosslinkable primary amino groups across the three domains of bonding are different. Reaction with diimidates causes a moderate loss of enzyme activity in all three enzyme types due to amidination rather than crosslink formation. On the other hand, crosslinking stabilizes the enzyme against urea and heat inactivation. This is most prominent with heterozygote acatalasemic catalase. Crosslinking markedly prevents the development of peroxidase activity that can be elicited in catalases by urea treatment. The role of the quaternary structure of the protein in the relationship between catalase and peroxidase activities is discussed.