Mummenbrauer et al. (1996) found that P53 protein exhibits 3'-5' exonuclease activity. This exonuclease activity is intrinsic to wildtype P53 protein, dependent on Mg2+, and it can be inhibited by addition of 5 mmol/L nucleoside monophosphates. In the present study, we intended to know whether mutated P53 protein--P53 delta 37 has this 3'-5' exonuclease activity or not. The results reveal that P53 delta 37 also has 3'-5' exonuclease activity. It has more protein and more exonuclease activity, but this activity can not be inhibited by guanine monophosphate. There are many processes related to exonuclease activity in mammalian cells; for example, DNA replication, DNA recombination and DNA repair. The exonuclease activity of P53 delta 37 protein may be important in these processes.