The monoclonal antibody 69A1 recognizes an epitope found on neurones with axons that fasciculate but not on those with non-fasciculating processes.

We have previously reported that the cell-type distribution and pattern of expression of the surface antigen recognized by the monoclonal antibody 69A1, suggests that it may be involved during the period of nerve fibre outgrowth and the formation of fibre bundles in the rat (Pigott & Kelly, 1986). In this current study, we have examined the expression of the epitope recognized by antibody 69A1 in regions of the rat central nervous system in which it is possible to distinguish between neurones with axons that fasciculate to form clearly defined fibre tracts and neurones with non-fasciculating processes. We have also examined antibody 69A1 labelling in several regions of the peripheral nervous system. We report that the 69A1 epitope is expressed on neurones with axons that fasciculate but is not found on neurones with short, non-fasciculating axons or on neurones without a morphologically identifiable axon. The antigen 69A1 has been purified and shown to be immunochemically closely related or identical to the L1 antigen.