To determine whether donor or recipient species influence calcification of bioprosthetic tissues, glutaraldehyde-treated valvar or pericardial specimens from different species (calf, sheep, pig) were subcutaneously implanted in different animals (rat, rabbit, cow, hen). Significant differences in the rate of calcification of the implanted specimens were found, which have important practical and theoretical implications for the development of valvular bioprostheses.