  Data from HERA, LEP and the Tevatron, as well as from low energy experiments
are used to constrain the Yukawa couplings for scalar and vector leptoquarks in
the Buchmuller-Ruckl-Wyler effective model. In the limit of very high
leptoquark masses constraints on the coupling to the mass ratio l/M are derived
using the contact-interaction approximation. For finite masses the coupling
limits are studied as a function of the leptoquark mass. Some leptoquark models
are found to describe the existing experimental data much better than the
Standard Model. Increase in the global probability observed for models
including S_1 or ~V_o leptoquark production/exchange corresponds to more than a
3 sigma effect. Assuming that a real leptoquark signal is observed, calculated
is an allowed region in the l-M plane. The leptoquark signal is mostly
resulting from the new data on the atomic parity violation in cesium, but is
also supported by recent LEP2 measurements, unitarity violation in the CKM
matrix and HERA high-Q^2 results.
