The ability of glutaraldehyde-treated tumor cells to induce protection against subsequent challenge has been studied in a syngeneic system. Two tumors have been tested in BALB/c mice. The first was a methylcholanthrene-induced tumor that has been maintained in serial passage for over a decade. The second was a spontaneous mammary adenoacanthoma that was tested at the third passage. The protection was found to consist of two components: (a) a specific immunological component; and (b) a nonspecific component observed when the immunizing and challenge dose were both given i.p. This nonspecificity, while possibly an element in tumor protective mechanisms, may confuse careful analytical studies of the immunogenic potential of tumor antigens.