Protein factors have been isolated from HeLa cells and from calf thymus which are able to specifically stimulate DNA polymerase alpha in vitro on templates which mimic the replication fork. One factor, extracted from HeLa cells, is an enzymatic complex of about 100-110 Kdal composed of a DNA-dependent ATPase and of an as yet uncharacterized DNA-binding protein. This complex exhibits a limited "helicase" activity on DNA : DNA partial duplexes which probably accounts for the stimulation of DNA polymerase alpha. The other stimulatory factor is obtained from calf thymus. They are the so-called single-stranded DNA binding proteins (DBP) which have a duplex-destabilizing activity. These proteins appear to be heterogeneous with regard to both physical properties (Mr and pI) and functional characteristics (DNA polymerase alpha stimulation, duplex denaturation). The origin and the biological significance of the different molecular forms are discussed.