Vaccination is one of the safest, most effective and cost-effective preventive interventions. The likelihood of achieving desired coverage is significantly increased by a more intense immunization effort focusing on at-risk low-income urban communities at the point of primary health care. Objective: The primary objective of this demonstration project is to examine the effect of provider-based interventions in private clinic to improve immunization coverage among low-income preschool-aged children, particularly Medicaid beneficiaries. Study Design: A randomized, controlled intervention trial will be conducted among selected private clinic serving a substantial number of preschool children <5 years of age who are enrolled in Medicaid Program. This prospective, experimental study design will compare the effect of two interventions among similar provider practice types. A control group for each practice type will serve to measure background in immunization coverage in the absence of new immunization practices. To measure effectiveness, the change from baseline immunization coverage 6 and 12 months after the intervention began will be computed as the primary outcome measures. Interventions. Two interventions will use: AFIX and Bonus. AFIX consists of periodic assessment of immunization coverage of children < 5 years of age in the practice who are enrolled in Medicaid and discussions with providers about policies, procedures and practices (feedback) as they relate to under immunization and improving the immunization coverage of all children in the practice. Data Analysis: Coverage levels (percentage of children up-to-date for immunization at baseline and 6 and 12 months after interventions) and rates of missed opportunities for immunization by provider intervention and control groups before and after the intervention will be calculated and compared.