Polyurethane foam plugs have long been known to be potentially toxic to algae and should be tested before use. We show that new, un-autoclaved diSPo R plugs are toxic to algae and that their toxicity varies with the pre-culture sterilization method used. This can make detection of toxicity difficult. Autoclaving a toxic polyurethane foam plug in an empty flask produced the greatest toxicity to Dunaliella sp. and resulted in the death of all cultures tested. We suggest using this treatment method when testing plugs for toxicity to algal cultures. Autoclaving a toxic plug in a fluid-filled flask was found to significantly decrease its toxicity to Dunaliella sp. After 9 days, algal cultures stoppered with toxic plugs which were initially autoclaved in a fluid-filled flask had similar observed color intensities but reduced cell densities compared to control cultures. Culturists using this sterilization procedure with toxic foam plugs may be unaware of potential problems due to the normal-appearing color intensity of the cultures. We recommend that the results of any studies using diSPo R and other potentially toxic plugs be re-examined for the possible effects of toxicity. This includes studies where foam plugs have been autoclaved in flasks containing culture water or media and the resultant cultures appeared normal.