This paper describes the synthesis of amphetamine derivatives, functionalised at the para-position of the phenyl ring of the drug. These derivatives were used to prepare fluorescein-labelled amphetamine to replace the label in a previously described polarisation fluoroimmunoassay that is highly specific for amphetamine. The original label was obtained using material donated by industrial sources and therefore not generally available. When the new label was used in the immunoassay, specificity was similar but the sensitivity was improved by a factor of four. This increase in sensitivity is explained by elimination of bridge-binding effects.