Abstract: The invention herein is directed to methods using Procion dyes to perform separations of interest in manipulating the NAD.sup.+ -independent ribotoxins. The methods are useful for preparing therapeutic agents containing these ribotoxins or their A polypeptide components. This separation method has been applied in particular to preparing hybrid toxins containing ricin toxins, both for purifying the resulting products and also for separating the components intended to be used in the preparation of these end products. In addition, a novel ricin isotoxin prepared using the method of the invention is disclosed.