The validity of the local-field approximation (LFA) has been tested in hemicyanine dye Langmuir-Blodgett monolayers. Resonant structures of absorption and the second-harmonic generation revealed the LFA is adequate to describe the second-harmonic generation in the monolayers even when the intermolecular interaction is very strong (H aggregate). A simple calculation procedure is also presented which enables on to treat monolayers as a macroscopic object in a consistent manner with the microscopic structure.