A line of recent theories and simulations have suggested that the nucleation of crystals might, under certain conditions, proceed in two steps: the formation of a droplet of a dense liquid, metastable with respect to the crystalline state, followed by ordering within this droplet to produce a crystal. Here, experimental tests are discussed of the applicability of this mechanism to the nucleation of ordered solid phases: crystals, or linear, planar, branched, or otherwise ordered aggregates, of proteins and small-molecule materials from solution. The main arguments stem from results on the kinetics of homogenous nucleation of crystals of the protein lysozyme. These results indicate that under a very broad range of conditions the nucleation of lysozyme crystals occurs via a modification of the theoretically postulated mechanism-as a superposition of fluctuations along the order parameters density and structure. Depending on whether the system is above or below its liquid-liquid coexistence line, a density fluctuation may never or may selectively lead to the formation of a dense liquid droplet; in the former case the high-density region, the "quasidroplet", is metastable also with respect to the dilute solution. In both cases, the molecules contained in the high-density region may attain an ordered arrangement, i.e., a structure fluctuation is superimposed on the density fluctuation and a crystalline nucleus obtains. This outlook on the nucleation of ordered solids from dilute phases suggests that the rate of nucleation can be controlled either by shifting the phase region of the dense liquid phase, or by facilitating the structure fluctuations within a dense liquid droplet or quasi-droplet. Results from literature indicate that the proposed two-step nucleation mechanism and the related tools for nucleation control may be applicable to the formation of crystalline and non-crystalline ordered solid phases of other, protein and non-protein materials, from solution. (c) 2004 Elsevier B.V. All rights reserved.