The growth of protein crystals is preceded by an induction period during which the protein solution prepares itself for crystallization. We have measured the length of the induction period for lysozyme as a function of the temperature for a solution of 16.9 mg/mL at pH 4.5 with 5% NaCl as the precipitating agent. The results are described with classical nucleation theory. The number of molecules in the critical nucleus n* turns out to be quite small: n* = 3-9. Such a small aggregate can hardly be considered as crystalline, as is assumed in the classical theory. By taking into account the anisotropy of the interaction between lysozyme molecules, we show that small nuclei, with n < 6, will have a linear, fiberlike structure. Larger nuclei have a more compact structure. The energy of small, linear nuclei smoothly joins the values for larger nuclei and large crystalline aggregates. The data measured for the induction time can be grouped into two regions, one with short and one with long induction times. The transition point is near 22 degreesC. This implies that the best starting point for crystallization of lysozyme under the conditions we used is at or just below this temperature. The transition point corresponds approximately to the change from linear to more compact nuclei.