The motile and rheologic properties of hamster lung and peritoneal macrophages (LMs and PMs) were examined by following the motions of magnetizable iron oxide (γ-Fe2O3) particles contained within phagolysosomes of these cells. As a measure of intracellular motility, γ-Fe2O3 particles in cells were magnetically aligned and the decay rate of the remanent magnetic field (RMF) in the direction of initial magnetization was monitored over time. Cytoplasmic rheology was measured by twisting the intracellular particles with a magnetic field (Btw) applied perpendicularly to the direction of initial magnetization. We measured changes in the RMF associated with application and removal of Btw. Intracellular motility in LMs and PMs was not significantly different (P > 0.20); similarly, cytoplasmic viscosity was not significantly different in LMs and PMs (P > 0.12); deformation on application of torque was significantly greater (P < 0.0001) and elastic recoil on removal of torque was significantly smaller (P < 0.0001) in PMs than in LMs; and by qualitative observation, the yield stress of cytoplasm (associated with a plastic, nonrecoverable deformation) was lower in PMs than in LMs. These results show that although cytoplasmic motion and viscosity are similar in the two cell types, PM cytoplasm is less stiff than LM cytoplasm as determined by yield stress.