Cryptochromes are FAD-based blue-light photoreceptors that regulate growth and development in plants and the circadian clock in animals. Arabidopsis thaliana and humans possess two cryptochromes. Recently, it was found that Arabidopsis cryptochrome 1 (AtCry1) binds ATP and exhibits autokinase activity that is simulated by blue light. Similarly, it was reported that human cryptochrome 1 (HsCry1) exhibited autophosphorylation activity under blue light. To test the generality of light stimulated kinase function of cryptochromes, we purified AtCry1, AtCry2, HsCry1, and HsCry2 and probed them for kinase activity under a variety of conditions. We find that AtCry1, which contains near stoichiometric amounts of FAD and human HsCry1 and HsCry2 (which contain only trace amounts of FAD), has autokinase activity, but AtCry2, which also contains stoichiometric amounts of FAD, does not. Finally, we find that the kinase activity of AtCry1 is not significantly affected by light or the redox status of the flavin cofactor.