This is a good question. There are many instances of prejudice in the play Twelve Angry Men. Some of the prejudice is subtle and some of it is very blatant. And as the play progresses the prejudice in the hearts of people becomes more explicit. Here are a few examples.
Juror ten is by far the most prejudiced. He says some very charged things.
We don't owe him a thing. He got a fair trial, didn't he? You know what that trial cost? ....Look, we're all grown-ups here. You're not going to tell us that we're supposed to believe him, knowing what he is. I've lived among 'em all my life. You can't believe a word they say.
Notice that he has an "us" vs. "them" mentality. So he makes certain pronouncements, simply because people live in a particular place. Generalizations, in this sense, can be very harmful. He is even more blatant in his prejudice in the following quotes.
"I don't understand...Look, you know how those people lie...They don't know what the truth is....Human life don't mean as much to them as it does to us...Look, these people are drinking and fighting all the time, and if somebody gets killed....They don't care."
There is no question of where Juror ten stands. There are also more subtle forms of prejudice, such as when Juror four says that the offspring of people from the slums are potentially all bad.
"The children who come out of slum backgrounds are potential menaces to society."