Peripheral blood eosinophilia of both allergic and nonallergic asthmatics was found to correlate with blood T cell activation and lymphokine production. A close correlation was shown between the increase of IL-2 receptor expressing T cells and the number of eosinophils. These in vivo activated T cells spontaneously released factors that prolonged eosinophil survival in vitro. The T cell derived lymphokines IL-5, GM-CSF and IL-3 were demonstrated to be responsible for prolonged eosinophil survival in vitro, and were identified in T cell supernatants and sera from asthmatics. In summary, T cell derived cytokines play an important regulatory function towards eosinophils in asthma.