1. Electron microscopy of solubilized photosystem II (PSII) complexes and PSII in spinach thylakoid membranes has been carried out and the results have been compared with data obtained from ordered two-dimensional arrays of PSII. Membrane-bound PSII is roughly rectangular (17.6 nm x 14.1 nm) with a central stain cavity surrounded by four major lumenal domains. A comparison between the averaged projections of single (non-ordered) particles at 3.8 nm resolution and the Fourier projection maps obtained from ordered arrays (at 2-3 nm resolution) reveals close similarity and excludes the possibility that PSII observed in two-dimensional ordered arrays represents an unusual subpopulation. 2. After detergent solubilization, PSII adopts various aggregation states which were analysed by electron microscopy in conjunction with single-particle averaging. Two different types of projection of roughly rectangular shape and of dimensions 30 nm x 17 nm manifesting themselves as tetrameric sandwich structures have been revealed. This conclusion is supported by the presence of at least two axes of 2-fold rotational symmetry running perpendicular to each other and intersecting at the centre of the oligomer. Comparisons of the structures of detergent-solubilized and native PSII show that the oligomerization of PSII can be artificially induced by the process of membrane solubilization.