These diminish in the north where rainfall rapidly filtrates into the subsoil. Despite extensive laboratory studies, which of these filtrates unionoids actually process remains uncertain. It is neutralized by antisera produced against toxic filtrates of "C. septicum" cultures through cross-neutralization. From crude filtrates of the "Cephalosporium acremonium" culture scientists got new antibacterial activity. They discovered that cell-free filtrates were sufficient to cause disease. This toxin is secreted, as it is isolated from filtrates of "C. histolyticum" cultures. The acidic filtrates are partially neutralized with sodium hydroxide to pH 3-4.