Extracellular metabolites from cultures of the marine green algaNannochloris sp. were extracted with chloroform. The combined extracts ("Aponin"). known to be cytolytic toward the red tide organism,Ptychodiscus brevis, were tested for activity against isolated canine tracheal smooth muscle. The extracts were without discernable effect on the contractile response (at concentrations up to 10.0 μg per ml). The absence of an effect, in this instance, is particularly significant inasmuch as the cytolytic activity of aponin could be responsible for release of airborne toxins ofP brevis, and these toxins may cause coughing, sneezing, and watering eyes in normal persons, as well as wheezing in asthmatic patients. The absence of an effect of aponin provides further evidence that aponin is a potentially useful red‐tide management agent.