Is it acceptable for me (an undergrad) to call professors and other research professionals by their first names? I'm a second-year undergrad, and I've always been on a first-name basis with anyone in academia or in a research setting. I first worked in a laboratory in high school, and since everyone was on a first-name basis with everyone else there, I sort of just applied that logic to other labs and to my college professors. 

But when I was talking to a researcher (my "supervisor", if that's relevant) at the lab I'm currently interning at this summer, one of my fellow interns told me afterwards she thought it was "odd" and "unprofessional" to call him by his first name. I understand where she's coming from, but if this were any other type of environment (say a business office), I think it would be awkward to call my boss anything but his first name (Mr. X seems a bit too formal for the boss-employee relationship). 

For what it's worth, my boss doesn't seem put off by the way I address him, and I don't think my professors are, either. Maybe my perspective's a bit skewed because of how I was introduced to the scientific/academic culture. Am I out of line here? What did you call your professors/advisors/bosses as an undergrad?
In all the years at the university, I only called the professors and lecturers by their last name and a doctor prefix. I also used to call them Professor + Last Name.

But never by their first names as far as they were older than me, they were professors and more important, it was not a custom in the universities I studied to call professors by their first names only (it was somehow impolite to call a professor by his first name only, even by other professors or colleagues).

It usually goes back to the custom. Sometimes the professors themselves prefer to be called by their first names. See how others call professors and see if it is polite or not. But my prefer is always Doctor + Last Name.