During formation of the neuromuscular junction, the basal membrane protein agrin initiates the aggregation of acetylcholine receptors (AChR) on the surface of myotubes. A muscle-specific kinase (MuSK) becomes phosphorylated upon incubation with agrin, although it does not bind to agrin on its own. Utilizing MuSK-specific antibodies, we demonstrate that the ability of different splicing variants and truncation fragments of agrin to trigger MuSK phosphorylation and AChR aggregation are correlated. Only agrin forms which are potent inducers of AChR-clustering are able to trigger the phosphorylation of MuSK. Picomolar concentrations of agrin are already sufficient to induce MuSK phosphorylation. Similar amounts are necessary for the aggregation of AChRs as well as their phosphorylation on a tyrosine residue. The complete overlap of specificities for MuSK phosphorylation and AChR aggregation suggests that only binding of agrin to a MuSK-containing receptor complex is responsible for the initiation of AChR aggregation. In contrast, interactions of agrin with binding proteins on the muscle surface harbouring different specificities such as alpha-dystroglycan do not seem to be necessary for this process.