Programmed cell death or apoptosis is a process in which unwanted cells are eliminated during growth and development. In mammals, several genes have been identified whose products are necessary to prevent entry into the apoptotic process. We have isolated a clone from an Arabidopsis thaliana cDNA library whose predicted translation product shows highly significant similarity to the mammalian defender against apoptotic death 1 (DAD1) protein. Transformation of the mutant hamster tsBN7 cells, which undergo apoptosis at restrictive temperature, demonstrates that the plant protein is as efficient as human DAD1 in rescuing these hamster cells from apoptosis. In contrast to mammals, Southern hybridisation and genomic data indicate that there are probably two genes in Arabidopsis thaliana. Northern blot analysis shows that AtDAD transcripts are present in all tissues examined, although the abundance of the transcripts is reduced in siliques during the maturation and desiccation phase of the seed. This is the first experimental proof that a homologue of an animal gene involved in apoptosis exists in plants and the first demonstration of complementation of a vertebrate mutant by a plant cDNA. Our results suggest that this process of suppression of apoptosis has been conserved in animals and plants.