Results of an experiment on the preservation of bioprostheses prepared from porcine aortal valves are presented. It is described how the structure of the cusps is affected by autolytic processes, by exposure to various concentrations of glutaraldehyde, and by sequential exposure to glutaraldehyde and papain solutions. Xenogeneic aortal complexes must be processed within the first 24 hours after collection since later on they undergo autolysis whereby the fibroblasts are destroyed and collagen fibrils become damaged. Low glutaraldehyde concentrations preserve the structure of these well enough, although concentrations below 0.5% were found to make the ground substance edematous and cause structural damage to the fibroblasts. Papain treatment without glutaraldehyde pretreatment leads to profound changes in the fibrous structures and in the cells, whereas glutaraldehyde fixation followed by papain treatment assures adequate preservation.