The universal Horn theory of relational Kleene algebra with tests (RKAT) is of practical interest, particularly for program semantics. We develop an (infinitary) proof system, based on well-founded trees of finite automata, which is sound and complete for this theory. A small modification of this system yields a proof system which is sound and complete for the universal Horn theory of ∗-continuous Kleene algebras with tests (KAT∗ ). This sheds light on the relationship between RKAT and KAT∗ .