During self-renewal of the hematopoietic stem cells there is a potential for these new cells to develop into abnormal malignant cells due to environmental, chemical or microbial challenges. This may lead to blood malignancies, including leukemia and lymphoma. Our laboratory was the first to develop a mouse model to study the development of lymphoma among pups of retrovirus infected mothers. We have established that if the newborn pups suckle breast milk from the mothers infected with a temperature sensitive Moloney Murine Leukemia Virus (MoMuLV-ts1), they develop lymphoma. Stem and progenitor cell populations were determined from blood, spleen, and thymus samples in infected mice with and without lymphoma using flow cytometry and specific protein markers, including CD117+/CD34+; CD117+/Sca-1+; CD135+/Sca-1+; Ter119+/CD71+ and CD117+/CD45+. Some stem cell populations were increased in infected mice with lymphoma. These stem cells appear to be involved in the development of lymphoma in our mouse model.