Factors affecting the absorption of immunoglobulin from colostrum by new born lambs were studied. Merino, Scottish Blackface, Cheviot, Southdown, Border Leicester, Finnish Landrace and Finnish Landrace x Dorset Horn lambs were removed from their mothers before sucking and bottle-fed with measured amounts of colostrum, either from individual ewes, or from pools of colostrum, some of which had been frozen for up to two years. Afterwards the lambs were fostered onto other ewes or reared on milk substitute. Their serum IgG1 concentrations before and after colostrum intake, were compared with IgG1 concentrations in conventionally reared lambs. The concentrations in lambs given a single feed of 150 to 200 g 1 hour after birth, or 30 g/kg at 1 hour and 7 hours compared favourably with those in control lambs. Lambs fed 50 to 100 g at 1 hour had a high proportion of very low concentrations and lambs fed 30 g/kg at 1 h, or at 1 and 24 h had low mean concentrations but no very low individual concentrations. The concentrations after a feed at 1 hour were positively correlated with the total amounts of IgG1 in the colostrum and negatively correlated with the birth weights. Only very small amounts were absorbed at 24 hours. Absorption was more efficient in males than in females and in Cheviots than in other breeds. It was unaffected by the age of the colostrum. Similar proportions of bottle-fed and control lambs died before 6 months of age, but relatively more bottle-fed lambs died from respiratory infections. The IgG1 concentrations and growth rates of the bottle-fed lambs were not significantly correlated.