If you're referring to the love theme from Romeo and Juliet, it is most likely still under copyright. I think it's from the 1930s? But someone else might know for sure. In the United States, anything written in 1923 or later is under copyright unless someone deliberately placed it in the public domain.
Okay, I did a search. Looks like, if it's the one I mentioned above, it's under copyright some places (like the U.S. and E.U., and it's not under copyright in some other places (such as Canada, China, Japan, South Korea). And it might depend on what you're using for your source for the melody. Copyright can get complicated. When using public domain material, I try to make sure I have an out-of-copyright copy of the music for reference.
Some songs (for example, "Scarborough Fair") are public domain, but many versions of them are not (like the one made popular by Simon and Garfunkel). So, it can get tricky. Keeps the copyright attorneys making a living.
This may or may not be helpful....