The latent hardening produced by straining copper crystals in single glide was investigated by measuring the flow stress on secondary systems. Specimens oriented for slip on these systems were spark cut from large prestrained crystals and tested in tension. These specimens did not always yield on the most highly stressed system; this behavior is a manifestation of the anisotropy introduced by the prestrain and was found only in certain portions of the stereographic projection.When the glide plane was changed, the flow stress increased. The increase ranged from 160% in early stage I to 40% in late stage II, except for systems with Burgers vectors at 90° to the prestrain slip direction, where systematically smaller increases were observed. The flow stress did not increase significantly when only the slip direction was altered.The anisotropy of the flow stress is closely related to the anisotropy of the dislocation distribution produced by single glide. In particular a reasonably good correlation appears to exist between the flow stress and the forest density. The strain rate and temperature sensitivity of the flow stress are unaffected by a change in stress system.The results are discussed in terms of current theories of the flow stress. It is concluded that while impurity interactions may be important in stage I, the flow stress is controlled mainly by forest interactions.