High-sensitivity radioimmunoassay for substance P

Optimisation of a non-equilibrium radioimmunoassay for substance P is described. The assay has a sensitivity of 2 pg (1.5 fmole), sufficient for substance P determinations in unexcepted human plasma and other biological material. The specificity of antisera used was tested with closely related peptides, physalaemin and eledoisin. Crossreaction of 1 per cent for eledoisin and 0.1 per cent for physalaemin was found. The assay is easy to perform, has a precision of 5 per cent, and takes only 24 hours.