The change in surface pressure (II) during adsorption of hemin, microperoxidase-8 (MP-8), microperoxidase-11 (MP-11), and cytochrome c at the air-water interface was studied using the Wilhelmy plate method. The surface activity decreases in the order hemin > cytochrome c > MP-8 > MP-11. It is suggested that the higher surface activity of MP-8, when compared to MP-11, is determined by the more pronounced anchoring of its heme group at the air-water interface. The measurements indicated instability of the surface films of microperoxidase-8 and -11. It is suggested that this effect is due to susceptibility of the substances to aggregation into the subphase. The adsorption rates (d Gamma/dt) of hemin and cytochrome c measured at constant area were in good agreement with those measured at constant II. This enabled application of the theory of an interfacial pressure barrier in the case of hemin and protein adsorption. The calculated area change per molecule on adsorption (Delta A) obtained for hemin indicated that the entire molecule needs to penetrate the surface film before adsorption in a perpendicular (or tilted) direction to the air-water interface. The Delta A value of 1.5 nm(2) for cytochrome c supports the conclusion that only a small portion of the protein molecule needs to enter the interface in order for adsorption to then continue spontaneously. Thus, the surface pressure studies revealed that systematic change in chemical composition of the complex protein molecule does not necessarily lead to systematic changes in interfacial properties.