A search has been made for molecules other than laminin that change their distribution during axonal regrowth after injury. Two monoclonal antibodies, generated against a protein extract of leech central nervous system (CNS), stain distinct regions of leech CNS and, following lesions, show changes in distribution with time. 1. Monoclonal antibody NP17 stained two bands of M(r) 80 x 10(3) and 60 x 10(3) on Western blots of protein extracted from whole CNS. On cryosections of leech CNS, staining was predominantly within the neuropile. 2. A second antibody, mAb CT16, stained a collagen-associated molecule of the extracellular matrix. It labelled the outer capsule, which surrounds the neuronal cell packets, and the inner capsule, which envelopes the neuropile. In the connectives, CT16 immunoreactivity was restricted to the connective capsule and was not associated with nerve fibres or glia. 3. When the connectives were crushed, immunocytochemical studies revealed changes in distribution of both mAb NP17 and mAb CT16. After 3 days, a time when fibres begin to sprout and form connections, an increase in NP17 immunoreactivity appeared at the site of the lesion. Staining levels remained elevated for several weeks. In contrast, CT16 immunoreactivity did not change for several days after damage. After 10 days, fibre-like staining appeared at the site of the crush; for several weeks it continued to spread throughout the connective. 4. These results show that regeneration of the leech CNS involves the redistribution of at least two molecules. Using monoclonal antibodies, these two molecules, which are situated in distinct regions of the CNS, have been visualized at different stages of the repair process. It has been shown that they alter their distribution at distinct times during regeneration.