This study examined the first-grade reading progress of children who participated in an intensive beginning reading intervention in kindergarten. Specifically, the study investigated whether kindergarten intervention could prevent first-grade reading difficulties, or produce an "inoculation" effect, for some children under certain instructional conditions. Participants included children at risk for developing reading difficulties who received a 7-month beginning reading intervention in kindergarten. In October of first grade, 59 children who had achieved criterion levels on measures of phonological awareness and alphabetic knowledge were randomly assigned to one of two types of first-grade reading instruction: (a) code-based classroom instruction and a supplemental maintenance intervention, or (b) only code-based classroom instruction. February posttest measures assessed oral reading fluency, word reading, nonword reading, and comprehension. Between-group analyses indicated that instructional groups did not differ on any posttest measure. The students' absolute levels of achievement were compared to national and local normative samples. These results indicated that between 75% and 100% of students in both conditions attained posttest levels and demonstrated growth comparable to their average-achieving peers. These results support the hypothesis that strong responders to kindergarten intervention can experience an inoculation effect through the middle of first-grade with research-validated classroom reading instruction.