Factors facilitating crystal nucleation and growth in biphasic systems, especially at the phase boundary are discussed. Biphasic systems of polyethylene glycol and ammonium sulfate were successfully used in the crystallization of tetragonal hen egg white lysozyme, which grew to half a millimeter in each dimension and diffracted to beyond 1.5 Å resolution. The high content of polyethylene glycol in mother liquor allows direct flash freezing of the crystals, essential for detector alignment and testing at synchrotron sources. The successful application of biphasic systems to produce high quality protein crystals makes them a useful alternative to monophasic experiments.