Revenge of the Creature, directed by Jack Arnold, from 1955.
Clint makes a brief appearance (uncredited) as a lab technician who is having trouble keeping track of his white rats.
Do you mean this to be a game of some sort, or did you really not know the answer? I ask because this is the "trivia games" forum, where we try to stump each other with Eastwood-related trivia questions, but we also have the "Questions & Answers" forum where people can post questions they don't know the answers to.