Polyclonal antibody-based enzyme-linked immunosorbent assays (ELISAs) were developed for the detection of retrorsine (1, 351 g/mol), monocrotaline (2, 325 g/mol), and retronecine (3, 155 g/mol) in the parts per billion (ppb) range. A set of three bifunctional linking arms (6-8) was synthesized. By N-alkylation of pyrrolizidine alkaloids (PAs) retrorsine, monocrotaline, and retronecine acetonide (9), six haptens (6.1, 6.2, 7.1, 7.2, 7.9, and 8.9) were synthesized and used to generate rabbit antisera. The resulting anti-retrorsine antiserum gave a 50% inhibition (I50) value of 0.9 +/- 0.2 ppb for retrorsine with detection limits of 0.5-10 ppb. The same ELISA system also detected isatidine (4, retrorsine N-oxide) dihydrate (403 g/mol) with an I50 of 1 ppb and senecionine (5,352 g/mol) with an I50 of 100 ppb. A second monocrotaline-based ELISA detected monocrotaline with an I50 of 36 +/- 9 ppb 2 with detection limits of 5-500 ppb and shows no cross-reactivity with 1 or 5; this ELISA demonstrates the potential for the substrate-specific detection method. A third retronecine-based ELISA detects 3 with an I50 of 3000 +/- 600 ppb (3 +/- 0.6 ppm) and detection limits of 600-10,000 ppb. None of these ELISAs cross-react with the structurally similar swainsonine (10) or lupinine (11) alkaloids. PAs were detected in extracts of Senecio vulgaris and Crotalaria retusa, but not in Lupinus spp., as a demonstration of the ELISA's usefulness.