The antigenicity of seven Schwannomas produced in pure-line Harvard rats by transplacental induction with ethylnitrosourea, was studied by transplanting tumour tissue subcutaneously or intracerebrally into neonate and adult Harvard rats. Tumour implants grew best in the neonate rat and probably grew better in brain than subcutaneous tissue, suggesting that the tumours were antigenic. Analysis of subcutaneous tumour growth in the neonate and adult rats suggests that not only were the tumours antigenic but their antigenic strength varied.