Homograft cell viability after cryopreservation was investigated and cytoimmunologic monitoring was performed during the early postoperative course to research possible immunologic reactions after allograft aortic valve replacement. After cryopreservation, morphologic observations were made, a nonradioactive cell proliferation assay was used, and prostaglandin I2 secretion of the remaining endothelial cells was determined. Cytoimmunologic monitoring was performed daily within the first 3 weeks postoperatively. An increase of the activation index greater than 1 was rated as an immunologic reaction. Maintained metabolic activity of graft endothelial cells after cryopreservation was confirmed by prostaglandin I2 release (9.24 +/- 3.48 ng/cm2 basic release and 20.1 +/- 5.76 ng/cm2 when stimulated with 25 mumol/L Na arachidonic acid). Cell proliferation was indicated after graft incubation with the nonradioactive viability kit (0.27 +/- 0.9 at 450 nm). Cytoimmunologic examinations (n = 861) after homograft implantation showed a more intense activation in patients with ABO-incompatible grafts (activation index 2.1 +/- 1.6, n = 16) than in those with ABO-compatible grafts (activation index 1.3 +/- 0.8, n = 17). In these groups, the duration of activation by cytoimmunologic monitoring was 2.8 +/- 1.5 days and 1.3 +/- 0.6 days, respectively (p < 0.041). No activation was observed in 8 patients after xenograft valve replacement (p < 0.01). Our data indicate that cryopreservation of homograft valves represents a cell- and tissue-protective preservation method. Postoperatively, all homograft valves caused immunologic reactions, which were reversible without immunosuppression treatment.