  If the transverse jet B fields observed in BL Lac objects on parsec scales
represent the toroidal component of the jet B fields, this should give rise to
rotation-measure gradients across the jets, due to the systematic change in the
line-of-sight component of the jet B field. We have found evidence for such
rotation-measure gradients in one-third to one-half of the 1 Jy BL Lac objects
we have analyzed. We present and discuss these new results, together with some
of their implications for our understanding of the parsec-scale jets of AGN.
