diff --git a/rs-tests.rkt b/rs-tests.rkt
index d7347e9..b185221 100644
--- a/rs-tests.rkt
+++ b/rs-tests.rkt
@@ -14,4 +14,6 @@
     (check-false (enabled? r1 s2))
     (check-equal? (list-enabled rs s1) '(a b))
     (check-equal? (list-enabled rs s2) '(b))
-    (check-equal? (union-products rs '(a b)) (set 'y 'z))))
+    (check-equal? (union-products rs '(a b)) (set 'y 'z))
+    (check-equal? (apply-rs rs s1) (set 'y 'z))
+    (check-equal? (apply-rs rs s2) (set 'y))))
diff --git a/rs.rkt b/rs.rkt
index 8bb6c39..23a78d7 100644
--- a/rs.rkt
+++ b/rs.rkt
@@ -10,7 +10,7 @@
  ;; Type names
  Species ReactionSystem
  ;; Functions
- enabled? list-enabled union-products)
+ enabled? list-enabled union-products apply-rs)
 
 ;;; =================
 ;;; Basic definitions
@@ -55,3 +55,9 @@
          ((inst set Species))           ; set-union requires at least one argument
          (for/list : (Listof (Setof Species))
              ([a as]) (reaction-products (hash-ref rs a)))))
+
+;;; Applies a reaction system to a set.
+(: apply-rs (-> ReactionSystem (Setof Species) (Setof Species)))
+(define (apply-rs rs s)
+  (let ([as (list-enabled rs s)])
+    (union-products rs as)))