Tumor suppressor gene product p53 in its wild-type conformation, is an effector of apoptosis. A rat histiocytic tumor, AK-5 which has a rearranged and mutated p53 gene undergoes apoptosis upon heat shock through surface expression of CD95 receptor. DNA sequence analysis of p53 gene from tumor cells revealed a deletion of 'C' at nucleotide position 942 and an addition of 'A' at position 1055. Deletion of one nucleotide caused premature termination of p53 protein which resulted in shorter p53 protein with an altered sequence from amino acids 315 to 341. Altered p53 was unable to protect BC-8, a single cell clone of AK-5 cells from apoptosis upon heat shock. BC-8 cells transfected with a wild-type p53gene (3B4 cells) were resistant to heat induced apoptosis and did not show the expression CD95 death receptor. Inhibition of p53 expression by using antisense oligo induced apoptosis upon heat shock in 3B4 cells. Similarly, inhibition of CD95 expression by antisense oligo inhibited heat induced apoptosis in BC-8 cells. In addition, cell cycle regulatory molecules, cdc2 and cdk2 are differentially regulated in a non-cell cycle dependent manner in these tumor cells. These results, in view of lack of heat shock response in BC-8 cells suggest a complex interaction between p53, CD95 and hsp70 which determines the fate of the cell. In the absence of functional p53, CD95 appears to be an effector of apoptosis in BC-8 cells.