diff --git a/ex4/nnCostFunction.m b/ex4/nnCostFunction.m
index 0cb283f..fd42d44 100644
--- a/ex4/nnCostFunction.m
+++ b/ex4/nnCostFunction.m
@@ -80,23 +80,14 @@ assert(size(J) == [1 1]);
 %               and Theta2_grad from Part 2.
 %
 
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+% Note: Theta1/2 are matrixes here, we want all their rows, but skip their
+% first column (not regularizing the bias term).
+regularization_term = lambda/(2*m) * ...
+                      (sum(sum(Theta1(:,2:end).^2)) ...
+                       + sum(sum(Theta2(:,2:end).^2)));
+assert(size(regularization_term) == [1 1]);
+
+J += regularization_term;
 
 % -------------------------------------------------------------