My employer allows a 100% pre-tax 401K contribution. This applies to regluar pay and/or bonus pay. However, if you always have to pay Social Security and Medicare tax which I believe is 7.65%, what does your check look like if you contribute 100%? Does anyone have any experience with doing this? I'm considering contributing 100% of a bonus but don't want to dip into my regular pay to make up for the payroll taxes.