The invention relates to particles for use in a scintillation proximity assays. In particular, the invention relates to particles for use in a scintillation proximity assay for detecting nitric oxide synthase activity in a sample. The invention also relates to a method of measuring nitric oxide synthase activity in a sample using these particles in a scintillation proximity assay. The invention further relates to methods using these particles to detect the presence of modulators of nitric oxide synthase activity.