diff --git a/src/Services/AdaptivePayments.php b/src/Services/AdaptivePayments.php
index 346c42f4..897020d7 100644
--- a/src/Services/AdaptivePayments.php
+++ b/src/Services/AdaptivePayments.php
@@ -27,7 +27,7 @@ private function setHeaders()
         $headers = [
             'X-PAYPAL-SECURITY-USERID'      => $this->config['username'],
             'X-PAYPAL-SECURITY-PASSWORD'    => $this->config['password'],
-            'X-PAYPAL-SECURITY-SIGNATURE'   => $this->config['secret'],
+            'X-PAYPAL-SECURITY-SIGNATURE'   => $this->config['signature'],
             'X-PAYPAL-REQUEST-DATA-FORMAT'  => 'JSON',
             'X-PAYPAL-RESPONSE-DATA-FORMAT' => 'JSON',
             'X-PAYPAL-APPLICATION-ID'       => $this->config['app_id'],
diff --git a/src/Traits/PayPalRequest.php b/src/Traits/PayPalRequest.php
index a9da1461..7aa37674 100644
--- a/src/Traits/PayPalRequest.php
+++ b/src/Traits/PayPalRequest.php
@@ -88,6 +88,13 @@ public function setApiCredentials($credentials, $mode = '')
             $this->config[$key] = $value;
         }
 
+        // Setup PayPal API Signature value to use.
+        if (!empty($this->config['secret'])) {
+            $this->config['signature'] = $this->config['secret'];
+        } else {
+            $this->config['signature'] = file_get_contents($this->config['certificate']);
+        }
+
         if ($this instanceof \Srmklive\PayPal\Services\AdaptivePayments) {
             $this->setAdaptivePaymentsOptions($mode);
         } else {
@@ -248,7 +255,7 @@ private function doPayPalRequest($method, $params)
         $post = [
             'USER'      => $this->config['username'],
             'PWD'       => $this->config['password'],
-            'SIGNATURE' => $this->config['secret'],
+            'SIGNATURE' => $this->config['signature'],
             'VERSION'   => 123,
             'METHOD'    => $method,
         ];