The effect of Ca(2+) ion interaction with monolayers of phosphatidylcholine (lecithin, L) was investigated at the air/water interface. We present surface tension measurements of lecithin monolayers obtained using a Langmuir method as a function of Ca(2+) ion concentration. The measurements were carried out at 22 degrees C using a Teflon trough and a Nima 9000 tensiometer. The interactions between lecithin and Ca(2+) ions result in significant deviations from the additivity rule. An equilibrium theory to describe the behavior of monolayer components at the air/water interface was developed in order to obtain the stability constants and area occupied by one molecule of LCa(+) and L(2)Ca complexes. The stability constants, K(1) = 1.92 x 10(3) m(2) mol(-1) and K(2) = 5.35 x 10(5) m(2) mol(-1) were calculated by inserting the experimental data. The value of area occupied by one LCa(+) complex is 65 A(2) molecule(-1), while the area occupied by an L(2)Ca complex is 117 A(2) molecule(-1).