In this work, isotherms for mixtures of methylene blue (MB(+)) and dimyristoyl phosphatidic acid (DMPA) were studied. Cc-spreading techniques were used to spread a mixed solution of MB(+) and DMPA in 3:1 CHCl3/CH3OH at the air-water interface. This system exhibits markedly expanded isotherms at low pi values. By using the Langmuir-Blodgett (LB) technique, a 1:1 DMPA/MB(+) monolayer was transferred to an indium tin oxide electrode at pi = 25 mN m(-1). The presence of MB + in LB films was detected by visible spectroscopy and electrochemical methods.