Transformation of a special population of non-adherent human lymphocytes by EB virus (EBV) is reversibly inhibited by co-cultivation on adult human fibroblasts. Neither fluid from adult fibroblast cultures nor extracts of fibroblasts inhibited such transformation, and the growth of already transformed lymphocytes was not inhibited on adult fibroblasts. The EBV-associated nuclear antigen (EBNA) was detectable in inhibited cultures, with a maximum level of about 15% at 14 days after which it decreased gradually. Reversal of inhibition at 28 days, by either addition of phytohaemagglutinin or by removal of the lymphocytes from the adult fibroblasts, resulted in a prompt increase in the percentage of EBNA-positive cells and typical lymphoblastoid outgrowth. Transformation of EBV-infected non-adherent lymphocytes could be inhibited by the addition of adult fibroblasts up to 2-4 days after infection. The results indicate that, in the EBV-infection of non-adherent lymphocytes on adult fibroblasts, a block resulting in inhibition of transformation occurs between the production of EBNA and the onset of autonomous proliferation of the infected lymphocytes.