Negatively stained electron microscope images are presented, showing the nucleation of two-dimensional (2-D) crystals of human erythrocyte catalase produced on mica by the negative staining-carbon film technique. Examples of the formation of partially ordered 2-D arrays and more ordered 2-D crystals are shown and the conditions required for the production of large well-ordered 2-D crystals discussed. The structural transformation of one flexuous 2-D paracrystal into a p2 2-D crystal is considered. The crystallographic 2-D image average of this p2 crystal form is presented (lattice parameters a = 9.0 nm, b = 18.6 nm, gamma = 90.8 degrees). It is shown that transmission electron microscopy provides the possibility of defining 2-D crystal nucleation, growth of intermediate forms, and low-resolution crystallographic structural analysis of 2-D crystals of human erythrocyte catalase. Comparison of the various electron microscopical negatively stained images with the peptide backbone of the X-ray structure of bovine liver catalase at different tilt and rotation positions correlates with and emphasizes the multiple intermolecular contacts and orientations that can be adopted by human erythrocyte catalase, leading to various 2-D arrays and 2-D crystals. Alignment of the surface groups involved in the protein-protein interactions that occur during 2-D crystal nucleation and crystal growth may ultimately be determined. From this approach, when taken together with detailed consideration of protein-solvent and protein-solute electrostatic interactions in solution, and at the fluid-air interface, it is considered that a more general theory of crystal nucleation and growth may eventually emerge. (C) 1995 Academic Press, Inc.