A retroviral vector has been constructed containing the mouse interleukin 4 (IL 4) gene under the transcriptional control of the thymidine kinase promoter. Infection of the IL 4-dependent T cell line CT4S by the thymidine kinase IL 4 construct resulted in factor-independent growth. The infected cells grow as a consequence of continuous consumption of the endogenously produced IL 4. Clones were established secreting different amounts of IL 4 but none of them was capable of growing in vivo. The lack of correlation between factor-independent growth and tumorigenicity distinguishes IL 4 from other growth factors and could reflect the recently reported activity of IL 4 to suppress tumor growth in vivo.