What was found
During climb of the flight from IAD to PIT, the crew reported that upon landing gear retraction they observed a right main landing gear "unsafe" red annunciator. The landing gear was cycled down, three green "safe" annunciators were observed, the aircraft returned to IAD and landed without incident. Maintenance personnel removed and replaced the right main landing gear uplocked indication microswitch. An operational check was satisfactory and the aircraft was returned to service.