Abstract: The invention provides compound which is an 18F-radiolabelled S-propylhomocysteine or a derivative thereof. The compound has an enantiomeric purity of at least about 90%. 18F-radiolabelled S-propylhomocysteine may be made by treating an N-protected ester of a substituted S-propylhomocysteine with a complexed F? salt in the presence of a base to form a protected product and then deprotecting the protected product to form the 18F-radiolabelled S-propylhomocysteine. In this method the N-protected ester has a leaving group on the S-propyl group and has an enantiomeric purity of at least about 90%. The base should be such that it does not cause racemisation of the protected product.