Cytotoxic effects of lymph and blood components, removed from sheep following renal allografting were determined by in vitro assay using 51Cr labelled donor sheep lymphocytes as target cells. Lytic antibodies were detected in lymph efferent from the node draining the graft site and at lower concentrations in blood serum and in lymph leaving the graft. The cytotoxic antibodies had both complement-dependent and leucocyte-dependent functions; the complement-dependent antibodies were fractionated and shown to be of IgM and IgG1 subclasses. Cytotoxic cells were found in lymph leaving the graft but not in blood or in lymph efferent from the draining node; those detected in the renal lymph were non-specific in action and their appearance correlated with increased numbers of macrophages.