Adult mice weighing 20 g, divided into 5 groups, were immunised with single and three cycle cryodestroyed ascites fibrosarcoma cells using different modes of immunisation with respect to dose and administration frequency. Survival against subsequent challenge with the same tumour cells and immune response given by leucocyte migration inhibition, were studied in these animals. It appears from the present study that animals with relatively low antigenic load have a significantly (P less than 0.001) high mean survival time and/or survival index compared with controls than those with relatively high antigenic load. Survival in animals immunised with three cycle cryodestroyed tumour cells is significantly (P less than 0.001) higher than that with single cycle even in face of a greater challenging tumour dose with mode of immunisation remaining the same. Preliminary observations on percentage leucocyte migration inhibition showed that there exists no difference in percentage inhibition with either viable or cryodestroyed tumour cell antigens added to migration chamber excepting in animals immunized with three cycle cryodestroyed tumour cells where percentage inhibition was significantly (P less than 0.001) greater with viable than with cryodestroyed tumour cell antigen without altering the percentage inhibition between the two groups. Factors maximising immune response and their modulating effects are discussed.