Although collagen-containing implants are widely used in various surgical applications, there has been relatively little attention paid to the possibility that this type of biomaterial may undergo pathologic calcification which could compromise its function. The present study reports for the first time the calcification of a series of implants of purified collagen sponges prepared with graded degrees of aldehyde-induced cross-linkages (assessed by shrinkage-temperature, wetting time, and collagenase digestibility). Type I collagen sponges were pretreated with either glutaraldehyde (0.1% to 2.0% aqueous solution, for 5-180 minutes) or formaldehyde (as vapors for 15 minutes to 15 hours), and implanted subcutaneously for 21 days in weanling rats. Although specimens not pretreated with either aldehyde reagent and the formaldehyde sponges pretreated for 15 minutes were resorbed without evidence of calcification, all other aldehyde-pretreated implants mineralized. The degree of calcification did not correlate with extent of cross-linking. Formaldehyde-pretreated implants calcified more extensively (Ca2+ = 87.8 +/- 2.8 micrograms/mg, mean +/- standard error of the mean; n = 58) than did glutaraldehyde-pretreated implants (Ca2+ = 40.9 +/- 1.4 micrograms/mg; n = 52). It is concluded that both glutaraldehyde- and formaldehyde-pretreated Type I collagen sponges calcify after subdermal implantation in young rats. Although aldehyde pretreatment of Type I collagen sponge implants is a prerequisite for their eventual mineralization, the threshold level of aldehyde-induced cross-linking required to potentiate their maximal pathologic calcification is low.