We propose to use ISO to carry out high-sensitivity spectroscopy
 of molecular bands around massive young stellar objects (YSOs).
 Infall and outflow are expected to take place simultaneously
 around these objects, and we believe that ISO equipped with
 the SWS and the LWS will be the best instrument in many years
 to study such processes by the means of IR spectroscopy.

 The outflows will be studied by the observation of the purely
 rotational lines of key molecules such as CO, OH, CH, CH+, and NH3
 in the spectral window of the LWS. We plan to observe several
 transitions to carry out a multiline analysis to obtain the
 physical parameters of the gas.

 The infalling gas will be studied by observing prominent
 bending bands of abundant molecules such as HCO+, HCN, HNC, HC3N,
 NH3, CCH, CO2, CS, N2H+, CH3OH, in absorption
 against the strong central IR sources by using the SWS. The
 absorption features are expected to be formed in the circumstellar
 region, and will provide crucial information about the infall
 processes taking place at the very vicinity of the young star.

 This project has been planned to complement the ISO Central
 Programme, which (besides some molecules such as H2O and H2) pays
 relative little attention to the molecular spectroscopy of the
 interstellar medium. The observations proposed here have been
 coordinated with other ISO proposals to study the emission of some
 other important molecules (C3, C4H2,H2O...) in the same sources. The
 results from these studies will provide a complete picture of these
 star-forming cores, and will be a major contribution to the IR
 spectroscopy of the interstellar medium.