Putrescine biosynthesis is elevated before DNA replication, and a stimulation of DNA synthesis by 20 mM putrescine has been found using an in vitro DNA synthesizing system. Furthermore, this stimulation of DNA synthesis by putrescine involves a particular factor (factor PA). This factor PA stimulates DNA polymerases alpha, beta, and gamma, and is present in nuclei and mitochondria but not in cytoplasm. Factor PA loses about 80% of its activity by heating at 45 degrees C for 15 min or by hydrolysis with 100 mg ml(-1) Enzygel trypsin. These properties indicate that factor PA is a protein. Its size is estimated to be about 2.1 S. DNA synthesis in nuclear and mitochondrial DNA polymerase extracts from tumour tissues and host livers of tumour-bearing rats are not stimulated by 20 mM putrescine. However, the addition of excess factor PA to DNA synthesizing systems using DNA polymerase extracts from proliferative tissues again results in a stimulation of DNA synthesis by exogenous putrescine. These findings indicate that the stimulatory effect of DNA synthesis in vitro by exogenous putrescine is controlled by the ratio between factor PA and endogenously synthesized putrescine in proliferative tissues or that sent by the bloodstream from proliferative tissues. These results suggest that a non-stimulatory effect of putrescine on DNA synthesis may be diagnostic in tumour-bearing patients.