Every endofunctor F of Set has an initial algebra and a final coalgebra, but they are classes in general. Consequently, the endofunctor F of the category of classes that F induces generates a completely iterative monad T. And solutions of arbitrary guarded systems of iterative equations w.r.t. F exist, and can be found in naturally defined subsets of the classes TY . More generally, starting from any category K, we can form a free cocompletion K of K under small-filtered colimits (e.g., Set is the category of classes), and we give sufficient conditions to obtain analogous results for arbitrary endofunctors of K. Key words: initial algebra, final coalgebra, completely iterative monad