<<I wish to claim may girlfriend as a dependent and file as a head of household. I understand she must meet 5 tests to qualify as a dependent. The only difficult one will be the support test (she is a full-time student under age 24, US citizen, no joint returns, and will live with me for all of 1998). >>

There are two separate tests here. One, HOH filing status, is NOT met because your girlfriend, supported or not, is not a qualfied person for this test. Let me know if you want the Code site for this.

Two, you may want to re-examine the relationship test for dependents. An unmarried live-in may qualify, but it depends on whether the relationship violates local law. Some states do not recognize common-law marriage and prohibit unmarried intimate relationships. Perhaps you've already looked at this and are comfortable with your state laws, but I thought I'd ask since if you fail here, the rest of your question is moot.