In this Letter we present calibrated observations of the average mass of galaxies with 22 < I < 25 in the Hubble Deep Field .
We measure the mean mass profile using the statistical gravitational lens distortion of faint blue background galaxies , based on 2221 foreground-background pairs .
The observed weak lensing distortion is calibrated via full 3-D simulations and other HST data on a foreground cluster of known mass .

Inside a projected radius of 5 ^ { \prime \prime } we find a 3 \sigma detection of shear .
Fitting to an isothermal mass distribution , we find a shear of 0.066 ^ { +0.018 } _ { -0.019 } at a radius of 2 ^ { \prime \prime } ( \sim 8 ~ { } h ^ { -1 } kpc ) .
The ( 1 \sigma ) limits on the parameters for the truncated isothermal model are \sigma _ { v } = 185 ^ { +30 } _ { -35 } km s ^ { -1 } , r _ { outer } \geq 15 ~ { } h ^ { -1 } kpc , and r _ { core } = 0.6 ^ { +1.3 } _ { -0.6 } ~ { } h ^ { -1 } kpc .
This corresponds to an average galaxy mass interior to 20 h ^ { -1 } kpc of 5.9 ^ { +2.5 } _ { -2.7 } \times 10 ^ { 11 } h ^ { -1 } M _ { \odot } .
Inside 10 h ^ { -1 } kpc , we find an average rest-frame V band mass-to-light ratio of 11.4 ^ { +6.0 } _ { -6.7 } h ( M / L _ { V } ) _ { \odot } .