We evaluated the relative impact of four procedures designed to encourage parents to obtain immunizations for their children. In a public health setting, the families of 1,133 immunization-deficient preschool children were randomly assigned to six conditions: (a) a general prompt; (b) a more client-specific prompt; (c) a specific prompt and increased public health clinic access; (d) a specific prompt and monetary incentives; (e) contact control; and (f) no contact control. All interventions, except the general prompt, produced some evidence of improvement when compared with the control groups. The monetary incentive group revealed the largest effect, followed by the increased access group, specific prompt group, and general prompt group, respectively. The data suggest that relatively powerful and immediate effects on preschoolers' clinic attendance for immunization may be produced by monetary incentives in conjunction with client-specific prompts. However, client-specific prompts alone appear to be the most cost-effective of the interventions.