Sticholysins are pore-forming toxins produced by the sea anemone Stichodactyla helianthus. When they encounter a sphingomyelin-containing membrane, these proteins bind to it and oligomerize, a process that ends in pore formation. Mounting evidence indicates that StnII can favour the activity of StnI. Previous results have shown that these two isotoxins can oligomerize together. Furthermore, StnII appeared to potentiate the activity of StnI through the membrane-binding step of the process. Hence, isotoxin interaction should occur prior to membrane encounter. Here, we have used analytical ultracentrifugation to investigate the oligomerization of Stns in solution, both separately and together. Our results indicate that while StnI seems to be more prone to oligomerize in water solution than StnII, a small percentage of StnII in StnI–StnII mixtures promotes oligomerization.