DNA synthesizing reactions catalyzed both large and small species of calf thymus DNA polymerase (DNA polymerase-alpha and -beta) [EC 2.7.7.7] were stimulated to comparable extents by the presence of spermidine or spermine, and it was not possible to differentiate these two species in terms of their sensitivities to polyamines. Optimal concentrations for stimulation were 0.5-1.0 mM for spermidine and 2-10 mjM for spermine. Excess polyamines strongly inhibited the reactions. The modes of stimulation were as follows: 1) Stimulation was observed with templates bearing long single-stranded sections when either natural DNA or synthetic homopolymer-oligomer duplex was used. 2) The nautral DNA-dependent reaction was stimulated by polyamines at suboptimal concentrations of Mg2+; the apparent Km value for Mg2+ was lowered on adding polyamines, while the Vmax value was unchanged. When synthetic homopolymer-oligomer duplex was used as a template, the reaction was stimulated spermidine even at the optimal concentration of Mn2+. 3) Polyamines markedly influenced the salt requirements of the reactions of DNA polymerase. Spermidine could replace salts such as KC1 or NaC1 at concentrations less than 1/100 of those of salts.