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Under most "whistleblower" laws, it depends on the employee's intent. (A whistleblower is an employee who complains of company misconduct, such as health and safety violations, shareholder fraud, or financial mismanagement.)
As long as the employee has a good faith belief that something illegal is going on, you cannot fire the employee for complaining about it. If, however, the employee knows the complaint is false and makes it simply to stir up trouble, the employee probably is not a protected whistleblower.
For information on whistleblower protections, including the Sarbanes-Oxley Act, see Nolo's article Punishing Whistleblowers Can Lead to Trouble.
For the most complete guide to your legal rights and responsibilities as an employer, get The Employer's Legal Handbook: Manage Your Employees & WOrkplace Effectively, by Fred Steingold (Nolo), which shows you how to comply with the most recent workplace laws and regulations, run a safe and fair workplace, and avoid lawsuits.