Collapsin-1/Sema III, a member of the semaphorin family, has been implicated in axonal pathfinding as a repulsive guidance cue. Cellular and molecular mechanisms by which collapsin-1 exerts its action are not fully understood. Collapsin-1 induces growth cone collapse via a pathway which may include neuropilin-1, a cellsurface collapsin-1 binding protein, as well as intracellular CRMP-62 and heterotrimeric G proteins. We previously identified a second action of collapsin-1, the facilitation of antero- and retrograde axoplasmic transport. This response occurs via a mechanism distinct from that causing growth cone collapse. To investigate the possible involvement of neuropilin-1 in the action of collapsin-1 on axoplasmic transport, we produced a soluble neuropilin-1 (sNP-1) lacking the transmembrane and intracellular region. sNP-1 progressively displaced the dose-response curve for collapsin-1 to induce growth cone collapse to higher concentrations. sNP-1 also inhibited collapsin-1-induced augmentation of both antero- and retrograde axoplasmic transport. Furthermore, an anti-neuropilin-1 antibody blocked the collapsin-induced axoplasmic transport. These results together indicate that neuropilin-1 mediates collapsin-1 action on axoplasmic transport. To visualize collapsin-1 binding to endogenous neuropilin-1, we used a truncated collapsin-1-alkaline phosphatase fusion protein (CAP-4). CAP-4 stains the growth cone, neurite, and cell body. However, local application of collapsin-1 to growth cone but to neither neurite nor cell body promotes axoplasmic transport. Thus, growth cone NP-1 mediates the facilitatory action of collapsin-1 on antero- and retrograde axoplasmic transport.