diff --git a/rs.rkt b/rs.rkt
index 4165870..b49841c 100644
--- a/rs.rkt
+++ b/rs.rkt
@@ -5,7 +5,7 @@
 
   (provide
    Species (struct-out reaction) Reaction
-   make-reaction)
+   make-reaction enabled?)
 
   (module+ test
     (require typed/rackunit))
@@ -26,6 +26,20 @@
     (test-case "make-reaction"
       (check-equal? (make-reaction '(a b) '(c d) '(e f))
                     (reaction (set 'b 'a) (set 'c 'd) (set 'f 'e)))))
+
+  (: enabled? (-> Reaction (Setof Species) Boolean))
+  (define/match (enabled? r s)
+    [((reaction r i _) s)
+     (and (subset? r s) (set-empty? (set-intersect i s)))])
+
+  (module+ test
+    (test-case "enabled?"
+      (check-true (enabled? (make-reaction '(a b) '(c d) '())
+                            (set 'a 'b 'e)))
+      (check-false (enabled? (make-reaction '(a b) '(c d) '())
+                             (set 'a 'b 'c)))
+      (check-false (enabled? (make-reaction '(a b) '(c d) '())
+                             (set 'b 'e)))))
   )
 
 (require graph "utils.rkt" "generic.rkt")
diff --git a/scribblings/rs.scrbl b/scribblings/rs.scrbl
index b5e21d2..ed5d439 100644
--- a/scribblings/rs.scrbl
+++ b/scribblings/rs.scrbl
@@ -59,6 +59,20 @@ instead of set syntax.
 (make-reaction '(a b) '(c d) '(e f))
 ]}
 
+@defproc[(enabled? [r Reaction] [s (Setof Species)]) Boolean]{
+
+A @racket[Reaction] is enabled on a set of species if all of its
+reactants are in the set and none of its inhibitors are.
+
+@ex[
+(enabled? (make-reaction '(a b) '(c d) '())
+          (set 'a 'b 'e))
+(enabled? (make-reaction '(a b) '(c d) '())
+          (set 'a 'b 'c))
+(enabled? (make-reaction '(a b) '(c d) '())
+          (set 'b 'e))
+]}
+
 @section{Org-mode interaction}
 
 This section contains some useful primitives for