Langmuir-Blodgett films of amphiphiles with an erotic acid (Oro, uracil-like) headgroup or a complementary adenine (Ade) headgroup were deposited onto mica surfaces and investigated by surface force measurement. Interactions between both complementary (Oro-Ade) and noncomplementary (Ade-Ade, Oro-Oro) nucleobase pairs were studied. The LB films showed functional features of the nucleobase headgroups. Electrolyte-dependent electrostatic repulsion was found at pH’s close to the pK(a) of the headgroups. In the absence of strong electrostatic forces, i.e., Oro-Ade and Oro-Oro combinations at pH 5.6, weak long-range attraction was detected. At close distances strong attractive forces made the surfaces jump into a contact position. The adhesive force between the complementary layers was found to be 100 mN/m, while the values between the noncomplementary pairs were ca. 50 mN/m and 250 +/- 50 mN/m for Ade-Ade and Oro-Oro, respectively. The force profiles found between nucleobase surfaces are made of two components. One is observed at ca. 100-20 nm and changes its intensity drastically depending on conditions. This force is found both for complementary and for noncomplementary surfaces and is similar to the very long range attraction reported for hydrophobic surfaces (Claesson et al., 1988, 1994; Kurihara et al., 1990, 1992; Evans, 1993). The other, appearing at around 20 nm, is characteristic of complementary pairs and remains unchanged for pH changes and by salt (KBr) addition. These forces may have an important bearing on interaction mechanisms involved in molecular recognition and supramolecular assembling in biological systems.