diff --git a/rs.rkt b/rs.rkt
index 70200c3..37ec509 100644
--- a/rs.rkt
+++ b/rs.rkt
@@ -5,7 +5,7 @@
 
   (provide
    Species (struct-out reaction) Reaction ReactionName ReactionSystem
-   make-reaction enabled? list-enabled union-products)
+   make-reaction enabled? list-enabled union-products apply-rs)
 
   (module+ test
     (require typed/rackunit))
@@ -77,6 +77,20 @@
                     (set 'z))
       (check-equal? (union-products rs '())
                     (set))))
+
+  (: apply-rs (-> ReactionSystem (Setof Species) (Setof Species)))
+  (define (apply-rs rs s)
+    (let ([as (list-enabled rs s)])
+      (union-products rs as)))
+
+  (module+ test
+    (test-case "apply-rs"
+      (define rs (hash 'a (make-reaction '(x) '(y) '(z))
+                       'b (make-reaction '(x y) '() '(t))))
+      (check-equal? (apply-rs rs (set 'x 'y))
+                    (set 't))
+      (check-equal? (apply-rs rs (set 'x))
+                    (set 'z))))
   )
 
 (require graph "utils.rkt" "generic.rkt")
diff --git a/scribblings/rs.scrbl b/scribblings/rs.scrbl
index 6f733a5..3ebaf6f 100644
--- a/scribblings/rs.scrbl
+++ b/scribblings/rs.scrbl
@@ -112,6 +112,17 @@ empty set.
                 '(a b))
 ]}
 
+@defproc[(apply-rs [rs ReactionSystem] [s (Setof Species)])
+		   (Setof Species)]{
+
+Applies the reaction system @racket[rs] to @racket[s].
+
+@ex[
+(let ([rs (hash 'a (make-reaction '(x) '(y) '(z))
+                'b (make-reaction '(x y) '() '(z)))])
+  (apply-rs rs (set 'x)))
+]}
+
 
 @section{Org-mode interaction}