  We report the first measurement of the parity-violating asymmetry in elastic
electron scattering from the proton. The asymmetry depends on the neutral weak
magnetic form factor of the proton which contains new information on the
contribution of strange quark-antiquark pairs to the magnetic moment of the
proton. We obtain the value $G_M^Z= 0.34 \pm 0.09 \pm 0.04 \pm 0.05$ n.m. at
$Q^2=0.1$ (GeV/c)${}^2$.
