This report concerns the influence of the thymus on the response to antilymphocyte serum (ALS) in newborn mice of the Balb/C strain. Thymectomy or sham surgery was performed within 24 hours of birth and the 120 involved mice divided into the following experimental groups on the basis of the material subsequently injected: isotonic saline, normal rabbit serum, antilymphocyte serum, neonatal thymectomy, neonatal thymectomy plus normal rabbit serum and neonatal thymectomy plus antilymphocyte serum. Parameters evaluated included spleen index, mortality, absolute lymphocyte count, renal lysozyme activity, primary and secondary hemolysin titers. The results suggest that, in contradistinction to adult mice, the consequences of ALS administration to newborns are but slowly reversible and complete recovery may not be possible in the absence of the thymus.