Enhanced tumor transplantability following second transplantation in mouse of original strain.

The XYZ factors have been prepared from tumor cells by keeping the tumor tissue frozen for a period of 10 days or longer, either by anaero bic storage (4, 8, 22) or by prolonged desiccation (lyophilization) (4, 24, 25). Ordinarily, tumor cells are killed by this treatment, although the presence of viable tumor cells does not neutralize or dimin ish the XYZ effect (4, 9, 16, 17). Furthermore, tumor cells have a different thermal death point from the XYZ factors (44° C. for the Z8352 tumor cells and 56° 0. for the Z8352 [23] and the Brown-