Extensive purification of DNA polymerase alpha-primase resulted in a marked loss of the DNA polymerase alpha activity. This loss is due partly to the elimination of some basic proteins from the enzyme preparation since the activity of purified enzyme was stimulated 10- to 15-fold by the addition of various basic proteins, including all five classes of histones, protamine, poly-L-lysine, and poly-L-arginine, at a concentration of 2 micrograms/0.2 ml in the presence of 20 micrograms/0.2 ml of activated DNA. The optimum concentration of the basic proteins and the maximum activity attained at that concentration varied with varying concentrations of the template primer used, indicating that the observed stimulation is caused by an interaction between these basic proteins and activated DNA. The enzyme activity with an optimal concentration of activated DNA was markedly inhibited by the addition of denatured DNA. The suppressed enzyme activity could be restored by an appropriate concentration of histone H1. These results suggest that histone H1 and other basic proteins protect the enzyme from forming an abortive complex with single-stranded DNA or with a long stretch of the single-stranded part of activated DNA as single-stranded DNA-specific binding proteins do (M. Sapp, H. König, H. D. Riedel, A. Richter, and R. Knippers (1985) J. Biol. Chem. 260, 1550-1556). Spermine also showed a similar stimulatory effect. All acidic proteins tested were ineffective.