The complete expression for the macroscopic second-order susceptibility (SOS) tensor chi((2)) for a monolayer with C-infinity symmetry at the air-liquid interface is derived as functions of the molecular SOS tensor alpha((2)) and the orientational order parameters. The chiral (non-chiral) terms of chi((2)) distinguished by their association with even (odd) order parameters, by which a second-harmonic generation (SHG) circular-dichroism effect can be better understood. An SHG experiment of a 4-cyano-4'-5-alkyl-biphenyl monolayer on an air-water interface by monolayer compression is also discussed.