The regulatory beta-subunit of the protein kinase CK2 modulates the salt optimum for alpha-subunit activity. In the presence of salt the beta-subunit is stimulatory while in the absence of salt it is inhibitory. In the presence of 150 mM NaCl CK2 has linear kinetics (Lineweaver-Burk) for the synthetic substrate RRRDDDSDDD with an apparent Km of 60 microM. In contrast, CK2 displayed biphasic kinetics for the peptide substrate when assayed in the absence of added NaCl. Biphasic kinetics were also obtained for other peptides but not for calsequestrin or casein. Recombinant alpha-subunit had strictly linear kinetics in the absence of added NaCl with an apparent Km of 104 microM. Preincubation of CK2 with ATP/Mg2+ or GTP/Mg2+, but not adenosine/Mg2+ or Mg2+ alone, resulted in kinetics that were near linear. This change in kinetics was dependent on enzyme conditions of low salt CK2 displays biphasic kinetics for peptide substrates, the biphasic kinetics require the presence of the beta-subunit, and ATP/Mg2+ binding reverses the effect.