Yahoo (via LaborProf Blog) reports that "One out of five (20 percent) U.S. workers claims that racial or ethnic discrimination exists in the workplace, according to a national survey by Hudson, reporting that they know of someone who has been denied a job, increased pay or promotion because of their ethnicity. That figure jumps to 31 percent for African-American workers, yet drops to 18 percent among white employees."
I hope you've updated your employee handbook lately and have trained your supervisors on how to deal with these situations.