We present here the results of carbon and nitrogen isotopic analysis of bone collagen undertaken on all skeletal remains of infants and young children below the age of 6 years (n = 34) from the internationally important British cemetery site at Wetwang Slack in East Yorkshire (middle Iron Age, ca. 4th to 2nd centuries BC). The aim of the study is to investigate infant diet, with particular reference to breastfeeding and weaning practices, and to compare the data with previously published studies of archaeological populations, particularly in the context of the variation in data patterns to be seen between sites. The skeletal remains from Wetwang Slack form the only prehistoric collection in the UK, prior to the Romano-British period, with sufficient individuals in this age group to make such an isotopic study viable alongside associated adults and older children. The data are compared in detail with published data from two other sites, one from 19th century Canada and the other from Medieval Britain. The results suggest an unusual situation at Wetwang Slack, with neither the nitrogen nor the carbon isotope ratios conforming to expectations when compared with the putative mothers. We discuss how these data compare with the expectation for breastfed infants and we interpret the divergence in this case to be due to restricted breastfeeding and the early introduction of supplementary foods. Am J Phys Anthropol, 2008. © 2008 Wiley-Liss, Inc.