Nude (athymic) or anti-lymphocyte serum-treated mice have absent delayed graft rejection due to impaired T-cell responses. Nonetheless, these mice can reject skin grafts, acutely, when treated with anti-H-2 antibody and additional complement. Resolution of the components in the H-2 antisera, by either absorption or by selective production of antisera of restricted specificity has demonstrated that anti-H-2K or H-2D antiser are graft destructive, and as shown elsewhere, are nonenhancing. By contrast, anti-Ia sera are not capable of causing allograft destruction even when used in extremely high doses and were previously noted to be enhancing. The mechanism of such differential effects is not apparent, but the findings are clearly of importance to transplantation in man, where sera reacting specifically with B cells may also be enhancing and nondestructive.