Affinity capillary electrophoresis (ACE) and pressure-assisted ACE were employed to study the noncovalent molecular interactions of antamanide (AA), cyclic decapeptide from the deadly poisonous fungus Amanita phalloides, with univalent (Li+, Na+, K+, and NH4+) and divalent (Mg2+ and Ca2+) cations in methanol. The strength of these interactions was quantified by the apparent stability constants of the appropriate AA-cation complexes. The stability constants were calculated using the nonlinear regression analysis of the dependence of the effective electrophoretic mobility of AA on the concentration of the above ions in the BGE (methanolic solution of 20 mM chloroacetic acid, 10 mM Tris, pH(MeOH) 7.8, containing 0-50 mM concentrations of the above ions added in the form of chlorides). Prior to stability constant calculation, the AA effective mobilities measured at actual temperature inside the capillary and at variable ionic strength of the BGEs were corrected to the values corresponding to the reference temperature of 25 degrees C and to the constant ionic strength of 10 mM. From the above ions, sodium cation interacted with AA moderately strong with the stability constant 362 +/- 16 L/mol. K+, Mg2+, and Ca2+ cations formed with AA weak complexes with stability constants in the range 37-31 L/mol decreasing in the order K+ > Ca2+ > Mg2+. No interactions were observed between AA and small Li+ and large NH4+ cations.