We have identified a small subset of CD3(+), CD4(+), CD8(-) thymocytes that do not express Thy1 (CD90). This Thy1(-) subset represents 1-3.7% of the total number of thymocytes in a naive mouse. CD4(+)Thy1(-) thymocytes express high levels of CD3, intermediate to high levels of heat-stable antigen (HSA), and low levels of CD25, CD45RB, CD69, CD44 and CD62L. They produce high titers of IL-4 and no IFN-gamma upon stimulation in vitro, a response characteristic of T(h)2 cells. In the thymi of mice infected neonatally with a high dose of the retrovirus Cas-Br-E MuLV, the frequency of CD4(+)Thy1(-) cells increased approximately 10-fold. High-dose virus infection resulted in decreased HSA and increased CD44 expression on CD4(+)Thy1(-) cells relative to cells from naive mice. CD4(+)Thy1(-) cells from high-dose infected mice also secreted IL-4 and not IFN-gamma upon in vitro stimulation. We previously reported that infection of newborn mice with a high dose of murine retrovirus results in the induction of a non-protective anti-viral T(h)2 T cell response; CD4(+)Thy1(-) thymocytes with a T(h)2-like cytokine profile may play a role in determining the cytokine bias of this anti-viral response.