Density-functional theory with interaction site model is applied to study binary mixtures consisting of Lennard-Jones atoms with hard-spherical cores (monomers) and bonded Lennard-Jones atoms of two hard-sphere sites (dimers). Strong amphiphilic interaction is shown to result in lamellae in the bulk condensed phase. A phase diagram showing disorder-lamellar phase boundaries and tricritical points is presented for a system with reduced interaction between the molecular species. Density distributions in the lamellar phase indicate that the mole fraction in the lamellae is determined by the concentration of the monomers.