Mice homozygous for the recessive allellc mutation motheaten (me) or viable motheaten (me’) on chromosome 6 develop severe defects in hematopoiesis. In this paper we present the findings that the me and me” mutations are within the hematopoietic cell proteintyrosine phosphatase (Hcph) gene. High resolution mapping localized me to an area tightly linked to Hcph on chromosome 6. Abnormalities of the Hcph protein product were demonstrated by Western blot analysis and by activity assays in both me/me and meylmev mice. Molecular analysis of the Hcph cDNA identified abnormal transcripts in both mutants. DNA sequence analyses of cDNA and genomic clones revealed that both the me and me’mutations are point mutations that result in aberrant splicing of the Hcph transcript. These findings provide the first available animal models for a specific protein-tyrosine phosphatase deficiency, thus facilitating determination of the precise role of this signaling molecule in hematopoiesis.