Massachusetts General Laws ch. 151B, §4:

It shall be an unlawful practice:

4. For any person, employer, labor organization or employment agency to discharge, expel or otherwise discriminate against any person because he has opposed any practices forbidden under this chapter or because he has filed a complaint, testified or assisted in any proceeding under section five.

4A. For any person to coerce, intimidate, threaten, or interfere with another person in the exercise or enjoyment of any right granted or protected by this chapter, or to coerce, intimidate, threaten or interfere with such other person for having aided or encouraged any other person in the exercise or enjoyment of any such right granted or protected by this chapter.