diff --git a/Makefile b/Makefile
index 7ed7aa2..2b1dc61 100644
--- a/Makefile
+++ b/Makefile
@@ -32,19 +32,19 @@ BRMSAPTOOL-DEPS=brmsaptool.scm testing.import.scm			\
 	listing.import.scm dictionary.import.scm month.import.scm	\
 	period.import.scm ansi.import.scm member-file.import.scm	\
 	command-line.import.scm members-base.import.scm			\
-	utils.import.scm
+	utils.import.scm primes.import.scm
 
 BRMSAPTOOL-SOURCES=brmsaptool.scm testing.scm listing.scm		\
 	dictionary.scm month.scm period.scm ansi.scm member-file.scm	\
-	command-line.scm members-base.scm utils.scm
+	command-line.scm members-base.scm utils.scm primes.scm
 
 BRMSAPTOOL-OBJS=testing.o listing.o month.o period.o ansi.o	\
 		member-file.o dictionary.o command-line.o	\
-		members-base.o utils.o
+		members-base.o utils.o primes.o
 
 BRMSAPTOOL-SHARED=testing.so listing.so month.so period.so ansi.so	\
 		member-file.so dictionary.so command-line.so		\
-		members-base.so utils.so
+		members-base.so utils.so primes.so
 
 brmsaptool: $(BRMSAPTOOL-DEPS)
 	$(CSC) -o $@ $<
@@ -133,3 +133,9 @@ UTILS-SOURCES=utils.scm testing.import.scm
 utils.so: $(UTILS-SOURCES)
 utils.o: utils.import.scm
 utils.import.scm: utils.so
+
+PRIMES-SOURCES=primes.scm testing.import.scm
+
+primes.so: $(PRIMES-SOURCES)
+primes.o: primes.import.scm
+primes.import.scm: primes.so
diff --git a/brmsaptool.scm b/brmsaptool.scm
index c1d83d5..f19d035 100644
--- a/brmsaptool.scm
+++ b/brmsaptool.scm
@@ -32,7 +32,8 @@
 	command-line
 	utils
 	ansi
-	members-base)
+	members-base
+	primes)
 
 ;; Print banner
 (print "brmsaptool 0.2 (c) 2023 Brmlab, z.s.")
@@ -48,6 +49,7 @@
 (member-file-tests!)
 (command-line-tests!)
 (members-base-tests!)
+(primes-tests!)
 (newline)
 
 ;; Command-line options and configurable parameters
diff --git a/primes.scm b/primes.scm
index d08f9b0..4385463 100644
--- a/primes.scm
+++ b/primes.scm
@@ -27,7 +27,10 @@
 
 (module
  primes
- (primes-tests!)
+ (
+  primes-tests!
+  is-4digit-prime?
+  )
 
  (import scheme
 	 (chicken base)