In MVC each controller action returns an "ActionResult". This can be a View, Content (plain text) or JsonResult for example. With JsonResult the output will (obviously) be serialized to JSON.
However - when using any serializeri on classes with bi-directional fields/properties you will run into problems as the serialization will lead to a endless loop trying to follow the bi-directional fields back and forth.
Using other serializer such as an XML serializer for example you just tag the bi-directional property with an "XMLIgnore" attribute and the serializer will basically ignore the field during serialization.
So, problem solved: Use the "ScriptIgnore" attribute to make the Json serializer in MVC ignore your bi-directional fields!