The present invention relates to a method for providing .alpha.-glucosidase inhibition to a subject by administering a pharmaceutical composition comprising a .alpha.-glucosidase inhibitory agent selected from pipataline (formula 1a), sesamin (formula 1b), pellitorine (Formula 1c), guineensine (Formula 1d) and brachystamide-B (formula 1e) having therapeutic application for diabetes mellitus, cancer, viral diseases such as hepatitis B and C, HIV, AIDS etc; also the invention provides a process for the isolation of said .alpha.-glucosidase inhibitory agent from the plant source Piper longum in significant yields.