This research investigates the determinants of family participation in the Aid to Families with Dependent Children-Unemployed Fathers (AFDC-UF) program, and projects the changes in caseload and cost that would arise under selected program modifications. The modifications include extending the UF program to all states, setting a nationwide benefit floor, and relaxing certain eligibility requirements. The analysis indicates that (1) the traditionally low UF caseload does not mean that the program is ineffective; (2) eligibility rules appear less influential than labor market opportunities and unemployment insurance in determining family participation in UF; (3) foreseeable modifications of the UF program would not cause drastic changes in caseload and cost; and (4) after alterations the UF program would still be less expensive and have narrower coverage than a negative income tax. (Author/JMB)