We measured the viscoelastic properties of the cytoplasm of J774 macrophages with a recently developed microrheometer. Ferromagnetic beads (1.3 microm in diameter) were used to determine the local viscoelastic moduli. Step-force pulses were applied to the magnetic beads and the displacement was observed by single particle tracking. By analyzing the creep response curves in terms of a triphasic mechanical equivalent circuit, we measured the shear elastic modulus, the effective viscosities, and the strain relaxation time. The values of the shear modulus vary by more than an order of magnitude within the cell population (range, 20-735 Pa; average, 343 Pa) and by a factor of 2 within single cells. The effective viscosity of the cytoplasm exhibits a relatively sharp distribution about an average of eta = 210 Pa s (+/- 143 Pa s). We measured the displacement field generated by the local forces by observing the induced motion of nonmagnetic beads. Even at distances of the order of 1 microm, no induced motion was seen, suggesting that the cytoplasm is composed of clusters of densely packed and cross-linked filaments separated by soft regions. In another series of experiments we analyzed the magnetophoretic motion of the ferromagnetic beads at a constant magnetic force. Measuring the bead velocity parallel and perpendicular to the applied force showed that local active forces on the beads varied from 50 to 900 pN.