We have observed the cold collision frequency shift of the 1S-2S trans
ition in trapped spin-polarized atomic hydrogen. We find Delta v(1s-2s
) = -3.8 +/- 0.8 x 10(-10)n Hz cm(3), where n is the sample density. F
rom this we derive the 1S-2S s-wave triplet scattering length, a(1s-2s
) = -1.4 +/- 0.3 nm, which is in fair agreement with a recent calculat
ion. The shift provides a valuable probe of the distribution of densit
ies in a trapped sample. [S0031-9007(98)07613-3].