A radioimmunoassay for the trypanocidal drug isometamidium (ISMM) has been developed using antibodies produced in sheep immunized against the drug cross-linked to human serum albumin. With 14C-ISMM as a radioligand, cross-reactivity of the antibodies was determined and found to be less than 3% with pyrithidium, diminazene and quinapyramine and about 13% with homidium. The limit of detection of ISMM in serum without prior extraction was 5.8 pmol/tube and within-assay and between-assay coefficients of variation were four and 14% respectively.