A new autosomal recessive mouse mutation, scat (severe combined anemia and thrombocytopenia), causes intermittent episodes of severe bleeding in the homozygote. At birth, affected mice are pale with intradermal petechiae and bruises on exposed surfaces. Central nervous system (CNS) bleeding occurs in 22% of the mice. Gastrointestinal (GI) hemorrhaging and splenomegaly are noted in moribund mice at autopsy. Of the 291 mice studied, 113 mice survived the initial crisis and entered a spontaneous remission period lasting from day 16 to day 27. A second crisis period ensued, and all but 22 mice died by 45 days. Mice in crisis show significantly decreased platelets, erythrocytes, and leukocytes and increased reticulocytes when compared to normal littermates. During remission all parameters are significantly improved or revert to normal values. Neither splenomegaly nor internal bleeding are observed during remission. A platelet-specific antibody is present in the plasma of mutant mice during crisis. The symptoms (severe bleeding, anemia, low- platelet counts) and platelet-specific antibody production are transferred to lethally irradiated normal mice through spleen cell transplantation. Splenectomy of mice in remission significantly increases survival. Exploitation of the features common to both scat/scat mice and patients with some forms of autoimmune thrombocytopenic purpura will undoubtedly prove useful in defining common pathways of disease development and in testing potential therapeutic measures.