Nucleation rate data for hen egg-white lysozyme crystallization were obtained using a particle counter. Tetragonal lysozyme crystals were expected to form at the temperature and solution conditions of these experiments: 4 degreesC, pH 4.5 with 0.1 M sodium acetate buffer and 2-6% NaCl (w/v). The rates varied as expected, as smooth monotonic functions of supersaturation at 2%, 3% and 6% NaCl. However, at 5% NaCl, a great deal of scatter in the data was observed. At 2% and 3% NaCl, all the batches contained crystals with tetragonal morphology. At 6% NaCl, almost all of the vials contained the white powder with few or no tetragonal crystals. At 5% NaCl concentration, a mixture of tetragonal crystals and powder formed in varying proportions in all the vials as observed by visual inspection. The powdery material was examined using optical microscopy and was seen to consist of needles with regular structure and sharp, faceted edges. Powder diffraction data from these needles was inconsistent with experimental powder diffraction data from tetragonal lysozyme crystals. It is possible that at high salt and protein concentrations liquid-liquid separation occurred and yielded a crystal polymorph.