A large number of passerine birds, mainly greenfinches, were found dead or dying in a hedgerow close to a field of onions recently sprayed with sodium monochloroacetate (SMCA). An analytical method is described for isolating monochloroacetic acid from bird tissues, as its potassium salt, by ion exchange chromatography. The ion exchange eluate is evaporated to dryness, acidified, extracted with ether and the ethereal extract methylated with diazomethane. The concentration of the methyl ester of monochloroacetic acid is determined using the Mass Selective Detector in the selected ion mode. Chemical analysis confirmed the exposure of the birds to SMCA. It is calculated that 50 microliter of spray contained the lethal dose of SMCA for a greenfinch.