The wall of the egg case of the dogfish (Scyliorhinus canicula) contains an analogue of collagen Types IV and VIII organised into a regular 3-dimensional network. It presumably provides both a protective and a filtering role for the eggs contained within it. Electron micrographs of longitudinal and transverse sections, including systematically tilted sections, have been used both to define, for the first time, the space group symmetry of the lattice and to carry out 3-D reconstruction of the unit cell contents. This cell was found to be tetragonal, space group I422, with a = b = 11 +/- 1 nm, and c = 74 +/-4 nm. Consistent with this, projection symmetries were c2mm for the [1,0,0] view p2mm for the [1,1,0] view, and p4mm for the projection down. the c axis (the [0,0,1] view), and all observed reflections in the computed Fourier transform obeyed the rule h + k + I = 2n (n integer) for body-centred lattices. The 3-D reconstruction, the first electron micrograph 3-D reconstruction of a collagen-containing material, is interpreted in terms of variations of previous molecular models for this structure. Type IV collagen is a constituent of the basal lamina, where it forms a network with both structural and filtering properties. The dogfish egg case structure may throw light on the (less regular) collagen IV structure of the basal lamina. (C) 1996 Academic Press