Abstract Lyngbya majuscula is a filamentous marine cyanobacterium with a worldwide distribution in temperate and tropical regions to a depth of 30 m. Over 70 chemicals have been isolated and characterised from this organism, many of which are biologically active. Previously, L. majuscula has been reported as implicated in negative health outcomes only in Hawaii and Okinawa. Recently large blooms of L. majuscula have occurred with increasing repetition in the Moreton Bay region as well as other areas along the Australian coastline. Lyngbya toxin A (LA) and debromoaplysiatoxin (DAT) were found in samples of L. majuscula collected from Eastern Moreton Bay and North Deception Bay, Queensland, Australia, respectively. Samples of L. majuscula were also obtained from West Maui, Hawaii and the freshwater Lyngbya wollei from Florida. A quantitative measure of the irritant effects of the chemicals found in L. majuscula was made using a mouse ear swelling test. The relative toxicities of two purified toxins, LA and DAT, were examined. These were found to produce swelling to a similar extent. The time course of inflammation and histopathological results were also similar for the two purified toxins. Less than 1 μg per ear of either toxin or a mixture (1:1) of the two toxins caused a measurable increase in ear thickness. When toxins were combined (1:1) there was an additive, not synergistic effect. Increases in ear thickness occurred within 15 min. Crude extracts of L. majuscula from Moreton Bay were also applied to mice ears. The effect of crude extracts from Eastern Moreton Bay was not fully explained by the measured LA content, suggesting other toxin(s) and/or modulating factors were present. The toxic effects of L. majuscula containing DAT from North Deception Bay were explained by the concentrations measured. Some samples of L. majuscula containing no measurable quantities of LA or DAT were found to exert an inflammatory response. This response had a different time course to the response produced by LA or DAT.