The most debilitating human lymphoid deficiency disease, known as severe combined immunodeficiency (SCID), impairs the differentiation of both T and B lymphocytes. Affected infants are highly susceptible to recurring infections of viruses, fungi and bacteria and invariably die within 2 yr of birth. Inheritance of this congenital syndrome may show X-linked or autosomal recessive control. To date autosomal recessive inheritance of SCID has been observed in Arabian foals which represent the only known animal model of this disease syndrome but here we report an autosomal recessive mutation in mice that severely impairs lymphopoiesis. Mice homozygous for this mutation have few if any lymphocytes; consequently they are hypogammaglobulinaemic and deficient for immune functions mediated by T and B lymphocytes. These mice, therefore, represent a new model for investigating how lymphoid differentiation may be impaired in the disease state and regulated in the normal state.