The effect of Ca2+ 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 Ca2+ 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 Ca2+ 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 L2Ca 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 angstrom(2) molecule(-1), the area occupied by an L2Ca complex is 117 angstrom(2) molecule(-1).