SummaryA comparison of serum immunoglobulin levels in children with Wiskott‐Aldrich Syndrotne (WAS) and in children with chronic eczema demonstrated lower IgM, higher IgA and lower IgG concentrations in WAS patients. Seven patients with WAS were studied for in vitro lymphocyte function by phytohaemagglutinin (PHA) stimulation and by one‐way mixed cell cultures. Lymphocytes from WAS patients were significantly depressed in their response to PHA when compared with lymphocytes from a control population of children of similar age. In contrast to this, lymphocytes from WAS patients and from their normal siblings and parents responded equally well to mitomycin‐blocked homologous