The chick biventer cervicis assay was found to be sensitive to samples that had produced unusually rapid responses in the regulatory mouse assay for DSP, the so called atypical response. Shellfish extracts originally produced by the regulatory laboratory (CEFAS) and fresh extracts produced from frozen samples were both active, demonstrating that the atypical response is not an artefact of the extraction process. The active extract produces a rapid and sustained contraction of the chick muscle preparation and a block of the twitch responses to stimulation of the motor nerve. Contractions in response to depolarisation of the muscle by KCl application are unchanged, indicating that muscle contractility is not directly affected by the extract. The initial contractures induced by the extract are not observed when neuromuscular transmission is blocked by tubocurarine, indicating that functional nicotinic acetycholine receptors are necessary for activity of the toxin. The active agent is thus most likely an acetylcholine agonist active on neuromuscular nicotinic receptors. The characteristic activity was found in a wide range of solvent extracts, suggesting that the compound has both polar and non-polar regions or that more than one molecule is involved. In ethyl acetate extractions, the activity was found to be restricted to a fraction that contained a compound having absorbance maxima at 216 nm and 280 nm. This peak appeared missing in inactive extracts of the same sample.