Mitochondria have emerged as central regulators of apoptosis. Here, we show that TID1, a human homolog of the Drosophila tumor suppressor lethal (2) tumorous imaginal discs, l(2)tid, encodes two mitochondrial matrix proteins, designated hTid-1(L) and hTid-1(S). These splice variants are both highly conserved members of the DnaJ family of proteins, which regulate the activity of and confer substrate specificity to Hsp70 proteins. Both hTid-1(L) and hTid-1(S) coimmunoprecipitate with mitochondrial Hsp70. Expression of hTid-1(L) or hTid-1(S) have no apparent capacity to induce apoptosis but have opposing effects on apoptosis induced by exogenous stimuli. Expression of hTid-1(L) increases apoptosis induced by both the DNA-damaging agent mitomycin c and tumor necrosis factor alpha. This activity is J domain-dependent, because a J domain mutant of hTid-1(L) can dominantly suppress apoptosis. In sharp contrast, expression of hTid-1(S) suppresses apoptosis, whereas expression of a J domain mutant of hTid-1(S) increases apoptosis. Hence, we propose that TID1 gene products act to positively and negatively modulate apoptotic signal transduction or effector structures within the mitochondrial matrix.