diff --git a/Makefile.am b/Makefile.am
index 777f35f1..d8a01574 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -338,12 +338,6 @@ test_run_tests_CFLAGS += -D_UNIX03_THREADS \
                          -qFLOAT=IEEE
 endif
 
-if OS400
-test_run_tests_CFLAGS += -D_ALL_SOURCE \
-                         -D_XOPEN_SOURCE=500 \
-                         -D_LINUX_SOURCE_COMPAT
-endif
-
 if AIX
 libuv_la_CFLAGS += -D_ALL_SOURCE \
                    -D_XOPEN_SOURCE=500 \
@@ -489,18 +483,6 @@ libuv_la_SOURCES += src/unix/pthread-fixes.c \
                     src/unix/os390-syscalls.c \
                     src/unix/proctitle.c
 endif
-if OS400
-libuv_la_CFLAGS += -D_ALL_SOURCE \
-                   -D_XOPEN_SOURCE=500 \
-                   -D_LINUX_SOURCE_COMPAT \
-                   -D_THREAD_SAFE
-uvinclude_HEADERS += include/uv/posix.h
-libuv_la_SOURCES += src/unix/aix-common.c \
-                    src/unix/ibmi.c \
-                    src/unix/posix-poll.c \
-                    src/unix/no-fsevents.c \
-                    src/unix/no-proctitle.c
-endif
 
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = @PACKAGE_NAME@.pc
diff --git a/configure.ac b/configure.ac
index ac6908fb..b2b80509 100644
--- a/configure.ac
+++ b/configure.ac
@@ -60,7 +60,6 @@ AM_CONDITIONAL([HURD],     [AS_CASE([$host_os],[gnu*],          [true], [false])
 AM_CONDITIONAL([LINUX],    [AS_CASE([$host_os],[linux*],        [true], [false])])
 AM_CONDITIONAL([MSYS],     [AS_CASE([$host_os],[msys*],         [true], [false])])
 AM_CONDITIONAL([NETBSD],   [AS_CASE([$host_os],[netbsd*],       [true], [false])])
-AM_CONDITIONAL([OS400],    [AS_CASE([$host_os],[os400*],        [true], [false])])
 AM_CONDITIONAL([OPENBSD],  [AS_CASE([$host_os],[openbsd*],      [true], [false])])
 AM_CONDITIONAL([OS390],    [AS_CASE([$host_os],[openedition*],  [true], [false])])
 AM_CONDITIONAL([SUNOS],    [AS_CASE([$host_os],[solaris*],      [true], [false])])