This chapter provides an overview of interleukin 3. IL-3 was initially defined as a factor present in conditioned media which induces the expression of the enzyme 20αSDH in cultures of splenic lymphocytes from athymic mice. With the availability of homogeneous IL-3 it became apparent that lymphokine is active in a variety of assays and is probably analogous to a number of factors defined by various assay systems. Among these it is possible to define assays specific for IL-3 and those in which IL-3 as well as other lymphokines are active. The specific assays include induction of 20αSDH and the ability to sustain the long-term growth proliferation of a variety of factor-dependent cell lines derived from long-term bone marrow cultures. IL-3 is also active in colony stimulating factor assays which detect several biochemically and serologically distinguishable factors. It also induces Thy 1 expression in cultures of bone marrow cells and is probably equivalent to a factor termed Thy 1 inducing factor. The chapter also explains assay of 20αSDH activity in lymphoid and hematopoietic organs.