Group Ia muscle spindle afferent fibres form giant terminals in Clarke's column which can be identified by morphological criteria. Postembedding Immunogold reactions were performed on tissue from the column using antiserum which recognized fixed L-glutamate in tissue. Giant terminals were heavily labelled with gold particles and quantitative analysis revealed that they contained significantly higher concentrations of L-glutamate in comparison with adjacent structures. L-Glutamate-enrichment of giant boutons is further evidence supporting the idea that this amino acid is a neurotransmitter at Ia synapses.