A three-dimensional (3-D) reconstruction of the keyhole limpet hemocyanin (KLH) didecamer has been obtained by single particle image analysis of cryoelectron micrographs. We exploit the random orientations of the individual particles within the vitreous ice matrix to reconstruct this oligomer in three dimensions without tilting the specimen holder and using only a single low-dose exposure of each specimen area. The symmetrical didecamer appears as a largely hollow cylinder of 370 +/- 5 Angstrom x 400 +/- 5 Angstrom and is apparently created by the face-to-face dimerization of two asymmetrical decamers. The molecule was reconstructed assuming D5 symmetry, which implies that a fivefold axis coincides with the cylinder axis and that five dyad axes lie within the central plane between the two decamers. The terminal domains of the subunit pair are thought to be folded into the central cavity at the ''collar'' region of tbe molecule, thus generating pronounced globular masses at each end of the cylinder. At the current level of resolution of about 45 Angstrom it is not yet possible to distinguish structurally between the two types of KLH didecamer, recently termed KLH1 and KLH2. (C) 1995 Academic Press, Inc.