The effect of varying the H-2 complex on the survival of parabiosed mice was investigated using a strain combination known to lead to about 50% long-term survival. While 45.6% of DBA/2J (H-2d) with (DBA/2J x C3H/HeJ)F1 (H-2d/H-2k) parabionts survive to 100 days or more, 69% of DBA/2J with (DBA/2J x C3H.NBSn)F1s (H-2d/H-2p) survive that long; only 16% of DBA/2J with (DBA/2Jx C3H.SWSn)F1s (H-2d/H-2b) survive to 100 days. The plot of hematocrits as a function of time after parabiosis differs significantly between the 45.6% surviving strain combination (which shows the classical pattern of parental polycythemia and F1 hybrid anemia) and the 69% surviving strain combination (which shows no divergence in parental and F1 hematocrit values). Furthermore, the 69% surviving strain combination often (5/14) persists in a chimeric state as judgec by starch gel electrophoresis of rec blood cell lysates; the other two strain combinations show no demonstrable red cell chimerism. The conclusion is suggested that the mechann these two generally successful strain combinations tested, and appears to be a function of the H-2types of the partners.