(1) The interaction of apocytochrome c with different molecular species of phosphatidylserine was studied using monolayers at constant surface area or constant surface pressure. The protein inserted readily into dioleoylphosphatidylserine monolayers up to a limiting pressure of 50 mN/m, whereas the interaction decreased with increasing molecular packing of the phosphatidylserine species, indicating the importance of the hydrophobic core of the lipid layer for the interaction. (2) The high affinity of apocytochrome c for dioleoylphosphatidylserine is indicated by the low Kd of 0.017 microM. There is little or no interaction with phosphatidylcholines. The importance of charge interactions is underlined by its ionic strength and pH dependency. (3) Experiments using 14C-labelled apocytochrome c indicate that cholesterol can enhance the protein binding. (4) It was demonstrated that apocytochrome c monomers penetrate the monolayer whereas oligomers can be formed in an adsorbed layer and washed off without changing the surface pressure. Preincubation of apocytochrome c in 3 M guanidine, to obtain the monomeric form, was essential to measure the full effect of interfacial interaction. (5) The molecular area of apocytochrome c changed from 1200-1300 A2/molecule in the absence of lipid to 700-900 A2/molecule after penetration of dioleoylphosphatidylserine monolayers. (6) Apocytochrome c-dioleoylphosphatidylserine interactions are only possible when the monolayer is approached from the subphase. It is concluded that the charge interactions are required for binding and penetration of the protein.