The electrophoretic migration of highly cross-linked carboxylated polystyrene latex spheres of 55, 140, and 215 nm radius (R) in solutions of linear polyacrylamide of 0.4 x 10(6), 0.6 x 10(6), and 1 x 10(6) molecular weight, in the 0.1-1% concentration range, was studied by capillary zone electrophoresis. The electric field strengths applied ranged from 40 to 530 V/cm. At the ionic strength used, these particles must be considered "large", exhibiting kappa R greater than or equal to 13 where kappa(-1) is the thickness of electric double layer. In the semidilute polymer concentration regime, the radius of the particles severalfold exceeds the average mesh size, xi, in the polymer network. It was found that particle retardation (expressed as mu/mu(0) where mu and mu(0) are particle electrophoretic mobilities in polymer solution and buffer alone) at a given polymer concentration decreases with both increasing particle size and electric field strength but increases with polymer molecular weight. The dependence of retardation on polymer concentration, c, follows a "stretched exponent", mu/mu(0) = exp(-alpha c＇＇). The prefactor a and the exponent nu are particle radius and electric field strength dependent. The microviscosity of polymer solutions defined as mu(0)/mu was well below values of zero shear viscosity measured viscometrically even when no dependence of microviscosity on electric field strength was observed. These findings were interpreted in terms of (i) a local shearlike deformation of the polymer network upon particle passage, resulting in a progressive decrease of the network entanglement density at the particle locales with particle translational velocity and, thus, a decrease of network resistance to particle penetration; and (ii) a progressive polymer depletion near the particle surface, with increasing particle radius at the scale of R/xi, which facilitates electrophoretic migration of the microparticle in the polymer solutions.