The cell adhesion molecule Tag-1 is highly expressed in immature cerebellar granule neurons (CGNs) during axonogenesis and is down-regulated prior to onset of radial migration. However, its precise role(s) during development of mammalian CGNs has been unclear. Here we studied the effects of anti-Tag-1 function blocking antibodies on the development of mouse CGNs in primary cell culture and in situ. Interfering antibodies inhibited axon formation by mouse CGNs in both cell cultures and in cerebellar slices. Effects on axon extension in cell cultures were observed under conditions of homotypic cell-cell contact, consistent with inhibition of cell adhesion activity. Further, when used as a substratum Tag-1 protein strongly stimulated neurite outgrowth by CGNs. Antagonism of Tag-1 also enhanced CGN migration in modified Boyden chamber assays. Radial migration was inhibited by Tag-1 antibodies in cerebellar slices, possibly reflecting a block in early CGN maturation in situ. These findings are consistent with a regulatory role for Tag-1 in axon emergence as well as migratory behavior by developing mouse CGNs.