Arent Jan Wensinck (1882—1939) is considered one of the founders of modern Islamic studies, and many researchers have relied on his conclusions. In one of his articles, he outlines a theory regarding the origins of Islamic purity laws according to which these laws were influenced by Jewish purity laws. In the article, he examines many issues relating to purity and impurity in Islam, albeit briefly. In this article I provide support for one of his arguments regarding the color of menstrual blood. Wensinck argued that on this issue, Islamic laws are identical to those of Judaism. I conclude that he was correct, with two minor qualifications.