We calculate Sudakov-type form factors for isolated spin-1/2 particles
(fermions) entering non-Abelian gauge-field systems. We consider both the on-
and the off-mass-shell case using a methodology which rests on a worldline
casting of field theories. The simplicity and utility of our approach derives
from the fact that we are in a position to make, a priori, a more transparent
separation (factorization), with respect to a given scale, between short- and
long-distance physics than diagramatic methods.Comment: 18 pages. RevTex is used. No figure
