Seven of nine primary 3:4 benzpyrene-induced fibrosarcomata in rats showed temporary slowing of growth or reduction in size following i.p. introduction of Millipore diffusion chambers containing lymph nodes from allogeneic rats, each specifically immunized with a biopsy from the tumour to be treated. Direct i.p. introduction of such lymph nodes did not alter the growth rates of the tumours. Millipore chambers containing lymph nodes from rats immunized with biopsies from tumours other than those being treated were without effect.