We demonstate a laser-cooled Cs fountain which can be used as a frequency standard. The fountain has been used to measure the frequency shift due to ultracold Cs-Cs collisions at temperatures below 2.8 \ensuremath{\mu}K. The measured shift is -12.9\ifmmode\pm\else\textpm\fi{}0.7 mHz for a density of (2.7\ifmmode\pm\else\textpm\fi{}1.5)\ifmmode\times\else\texttimes\fi{}
109 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ beginning with an even distribution (\ifmmode\pm\else\textpm\fi{}10%) among the F=3,
mF states. For a fountain with 95% of the atoms in the
mF=0 states, we measure a frequency shift of -5.5\ifmmode\pm\else\textpm\fi{}0.5 mHz at a density of (3.5\ifmmode\pm\else\textpm\fi{}2.0)\ifmmode\times\else\texttimes\fi{}
108 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$.
In our Cs atomic fountain, we laser cooled and trapped 10(exp 10) Cs atoms in 0.4 s. By shifting the frequencies of the laser beams, the atoms were launched upwards at 2.5 m/s and a fraction of the atoms were optically pumped into the F=3 ground state. The unwanted atoms in the F=4 ground state were removed from the fountain with radiation pressure from a laser beam tuned to excite only those atoms. The Cs atoms in the F=3 state traveled ballistically upwards, were excited by the microwave cavity, and then returned back through the same cavity in the atomic fountain configuration.