Langmuir-Blodgett (LB) films of the reaction center protein-pigment complexes (RCs) isolated from photosynthetic bacteria Chloroflexus aurantiacus,, Rhodobacter sphaeroides and Rhodopseudomonas viridis were prepared and investigated by small-angle X-ray reflectivity. The patterns obtained for the eight-monolayer (LB) films of the RCs from Rb. sphaeroides and Rps. viridis show well defined Kiessig fringes. This confirms the preparation of molecularly smooth (roughness less than 1.5 nm), flat RC films and allows us to estimate their thickness as 30.5 and 34.0 nm respectively. Well ordered RC deposition took place only onto the "front" side of the substrate, which faces the moving barrier in the Langmuir trough. X-ray reflectivity measurements of LB films of RCs from C. aurantiacus show no Kiessig fringes, independent of the side of the glass plate; this can be explained due to the formation of an unordered protein film during LB deposition. These effects are due to the high asymmetry of the RCs from Rb. sphaeroides and, especially, Rps. viridis in comparison with the RCs from C. aurantiacus.