  Evidence is mounting that many of the transverse jet B fields observed in BL
Lac objects on parsec scales represent the dominant toroidal component of the
intrinsic jet B fields. If this is the case, this may give rise to
rotation-measure (RM) 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 RM gradients in several BL Lac objects. We discuss these new results,
together with some of their implications for our understanding of the pc-scale
jets of AGN.
