The response of various cell types to first-set and secondary allografts in S. plicata was tested. The data implicate lymphocyte-like cells in allograft rejection. They were the only cells to specifically invade incompatible allografts and the surrounding tissue prior to rejection. This influx coincided with the destruction of other cells within allografts and thus may have been responsible for graft loss. Other cell types did, however, undertake non-specific responses to grafts which were probably stimulated by wounding. These results are discussed in terms of the evolution of the vertebrate immune system.