Anatomical distribution of common acute lymphoblastic leukemia antigen (CALLA) was studied in lymphomas as well as in normal lymphatic organs using the monoclonal antibody VIL-A1. Twelve lymphomas were labelled by VIL-A1. Three of the 12 tumours also had T-cell marker, six lymphomas also showed immunoglobulin staining and only three tumours were pure CALLA lymphomas. Tonsils showed a distinct CALLA labelling of many germinal centre cells and of singular cells in interfollicular T-cell regions. Children's thymuses showed rare distinctly labelled cells in the cortex and medulla and slightly more cortical cells stained faintly by VIL-A1. Foetal thymuses of about the twelfth week of gestation contained many heavily labelled cells. The findings are discussed as evidence for the presence of CALLA on immature B as well as T lymphocytes. They favour the idea of CALLA as a common lymphocyte differentiation antigen although other possibilities of interpretation are also discussed.