  We report the discovery of two very cold and massive molecular cloud cores in
the region ISOSS J18364-0221. The object has been identified by a systematic
search for very early evolutionary stages of high-mass stars using the 170
micron ISOPHOT Serendipity Survey (ISOSS). Submm continuum and molecular line
measurements reveal two compact cores within this region. The first core has a
temperature of 16.5 K, shows signs of ongoing infall and outflows, has no NIR
or MIR counterpart and is massive enough (M ~ 75 M_sun) to form at least one O
star with an associated cluster. It is therefore considered a candidate for a
genuine high-mass protostar and a high-mass analog to the Class 0 objects. The
second core has an average gas and dust temperature of only ~ 12 K and a mass
of M ~ 280 M_sun. Its temperature and level of turbulence are below the values
found for massive cores so far and are suggested to represent the initial
conditions from which high-mass star formation occurs.
