Attempts were made to prolong the survival of normal tissues by the passive transfer of humoral isoantibodies directed against H-2 isoantigens. It was demonstrated that strain (A X A.CA) F1 skin grafts survived for 10 days in untreated A.CA recipients and for 11.5 days in repeatedly antiserum treated (A.CA-anti-A serum) hosts. This strain combination involved non-H-2 differences in addition to those determined by the H-2 complex. Skin graft survival was more prolonged, as a result of antiserum treatment in another combination, where donor and recipient differed at the H-2 locus only. B10 skin grafts survived for 13.4 days in antiserum treated (B10.D2-anti-B10 serum) recipients and for 10.6 days in untreated controls. More pronounced prolongation of graft survival was obtained with ovaries. (A X A.CA) F1 ovaries had a median survival time of 19 days (mean 25.0 ± 12.8 days) in A.CA recipients treated with A.CA anti-A serum compared to 0 days (mean 5.7 ± 3.9 days) in untreated controls. Four out of 25 H-2-incompatible ovaries functioned for more than 49 days in antiserum-treated hosts.