Several in vitro investigations and animal experiments are described which may be used as experimental basis for the enzymatic treatment of rheumatoid arthritis and, possibly, also other immune complex diseases. Demonstration of absorption of unaltered orally-administered radiolabelled enzymes is shown in guinea pigs and rabbits. In vitro experiments with 4 types of soluble immune complexes which were incubated with gradually increasing amounts of enzymes showed dose-dependent cleavage of complexes. Antigen-induced experimental arthritis of rabbits, fed different amounts of a therapeutically used mixture of enzymes at different times, could be inhibited by this treatment, in dependence of dosage and time of feeding. With respect to the therapeutic applications of this study, the results favour the use of a high dosage repeated daily administration, since duration of effect seems limited.