Effect of C. Parvum on immunization with irradiated tumour cells.

S.c. injection of tumour cells or small pieces of tumour irradiated to a dose of 22,000 rad evoked resistance to live challenge with the same tumour (a CBA strain fibrosarcoma induced with methylcholanthrene) 14 days later. This resistance was, however, over-ridden if the challenging inoculum was sufficiently large, and did not develop if the cells were irradiated to 100,000 rad. The resistance evoked by injection of 10(6) irradiated tumour cells was impaired by i.p. injection of 1-4 mg C. parvum 5 days before, and virtually abolished by a similar injection 11 days after, the irradiated cells. The effect of s.c. injection of a mixture of 10(6) irradiated cells and C. parvum 14 days before live challenge depended on the dose of C. parvum. With 0-7 mg the development of resistance was largely but not completely abrogated; 0-35 mg resulted in a lesser degree of abrogation, and 0-09 mg or 0-02 mg had little or no effect.