This study used immunoelectron microscopic techniques to define the ultrastructural location of MAGP-1 on the fibrillin-containing microfibrils of the ocular zonule. A specific anti-MAGP-1 monoclonal antibody (MAb), 11B, was produced that did not crossreact with fibrillin-1 or other microfibrillar proteins. MAb 11B was shown by immunofluorescence to localize intensely to zonular tissue. Postembedding immunoelectron microscopy showed that MAGP-1 was associated with microfibrils throughout the zonule, with the exception of a narrow band of microfibrils at the junction with the lens capsule. With preembedding labeling, the anti-MAGP-1 MAb was found to localize in a crossbanding pattern, at intervals of about 50 nm, to microfibrils throughout the zonule and along bundles of microfibrils in surrounding vitreous tissue. Rotary shadowing of isolated microfibrils showed a "beads on a string" morphology with a periodicity of about 50 nm. With immunogold labeling, the anti-MAGP-1 antibody specifically localized on the beads in a symmetrical manner. Occasionally two gold partides were attached to the same bead, suggesting that multiple MAGP-1 molecules were present in the structure. The results indicate that MAGP-1 is intimately and regularly associated with the bead regions of fibrillin-containing microfibrils. The findings are consistent with a major structural role for MAGP-1 in microfibril biology.