We present evidence for a coherent shear signal in a field containing the z=1.2 radio-source PKS1508-05. Since there were no intervening mass concentrations known before targeting this field, we interpret this signal as due to weak lensing by large-scale structure. This result is the outcome of a re-analysis of the observations of Fort et al. (1996) in the fields of three high redshift QSO/radio-sources. Several tests of the robustness of the signal were performed: subdivision of the field and tests using simulated data with randomized ellipticity orientation; using the pixel-to-pixel autocorrelation function method to measure the shear; using the correlation function of the galaxy ellipticities with bootstrap resampling to estimate errors and to find the typical angular separation that dominates the signal. We find that the field of PKS1508-05 contains a robust shear signal of about 3% that is coherent over the 2' by 2' field. The signal is detected at a high level of significance (SchlangeUnter > 99.6%) using the tests described above. Combining the three QSO fields, we find an rms shear of about 2.2% on the scale of a 2'x 2' field, however with a large uncertainty which is dominated by cosmic variance. This value is close to that expected from the cosmological mass distribution on that scale.