Legal Question in Employment Law in California

"Illegal"?

IF, IF, you can get detailed testimony from such prospective employers as to the 'specific' things being told to them about you, and IF those are things are provably and knowingly false as to the facts, and those false statements were the reason you were rejected for employment, then you would have a claim that could be the basis of a lawsuit. Keep in mind that a statement of opinion such as 'he was a lousy employee that we were happy to see leave' is not actionable.

I advise people to hire an attorney to write a letter to the old employer advising that you believe they are slandering you, and that unless they immediately correct that situation, you would be compelled to take legal action. You can't write such letter yourself and expect it to work.

Afterthought:

If you were 'fired', you may reasonably assume they really didn't like you. Smart companies will 'lay off' people to minimize the anger response.

You need to understand also, that an employer has the same free speech rights as you, including to tell the truth about you and your performance, attitude, etc., even if you were fired 'for cause' or 'misconduct'.