A high frequency of cytotoxic precursor cells for a syngeneic tumour.

The average frequency of cytotoxic precursor cells in DBA/2 lymph node cell preparations reactive to the syngeneic tumour P815 has been determined as 1 in 2000. This frequency is similar to the precursor frequency for an allogeneic tumour EL-4. The normal lack of response of DBA/2 lymph node cells to the syngeneic tumour P815 in vitro cannot be attributed to a lack of cytotoxic precursor cells. We conclude that in this tumour-host system non-immunogenicity reflects a defect at the inductive step.