Found the answer to my search with some help from someone on IMDB. The film I was looking for with the Jekyll & Hyde plot is "The Vampire" (1957) starring John Beal. The plot is typical Jekyll & Hyde, but the doctor/scientist in the film is experimenting with bats and thus the serum transforms him into a vampire-like villain. But, not the typical classy, graceful, agile vampire, rather, a crude, clumsy killer/bloodsucker. Can't wait for the DVD in the mail. Thanks for all the replies. Cheers and Happy New Year!