Consumption of solid feed is essential for successful transition from a preruminant to a functional ruminant animal. Calves fed starter rations containing highly fermentable carbohydrates often experience dramatic changes in concentrations of rumen and blood metabolites. The optimal amount of roughage required in the diet of young calves is still unclear. The objective of these studies was to determine the effect of form of diet and the inclusion of various levels of hay of consistent particle size on rumen development and immune function in young calves. In experiments 1 and 2, calves were fed one of four treatments. Diets consisted of coarse starter (C), ground starter (G), coarse starter with 7.5% grass hay of consistent particle size (H1), and coarse starter with 15% grass hay of consistent particle size (H2). When intake was restricted prior to weaning, calves consuming H1 and H2 tended to have greater body weight. Calves receiving H1 and H2 also had greater average daily gain (ADG) and feed efficiency (GF) postweaning. Intake of dry matter did not differ by treatment. Total ruminal VFA concentrations were higher in calves consuming G versus C. Acetate to propionate ratio was higher in calves fed H2 than H1. In experiment 2, calves received access to feed ad libitum. There was a tendency for calves offered H1 and H2 to consume more starter and total dry matter than those fed C. There were no differences in body weight, ADG, GF, or age at weaning. In experiment 3 calves were offered spray-dried animal plasma (SDAP) or red blood cells and whey protein concentrate (RBCW) in addition to coarse and ground diets in experiment 1. Calves receiving diets supplemented with SDAP were heavier. Changes in white blood cell populations were also observed, although it is doubtful these changes are of physiological significance because the values did not differ from normal ranges. It appears that the use of forage of a consistent particle size does encourage intake of solid feed and promotes more efficient gain. The use of hay in the starter diet appears to create a more stable rumen environment for the calf.