The efficiency of transformation of human lymphocytes after infection with Epstein-Barr virus (EBV) was determined in fractionated and non-fractionated preparations derived from 16 human cord blood samples and two blood samples from adult donors. The transformation efficiency of macrophage-depleted leukocytes was consistently lower as compared to non-fractionated leukocytes. Additional depletion of B-cells resulted in a further decrease. Reduction of T-cells, however, did not influence significantly the transformation rate. In non-fractionated leukocyte cultures, as well as in macrophage-depleted and B-cell enriched cultures, colonies of transformed cells were regularly observed within the first week of cultivation. All cell lines established after EBV-infection revealed membrane-bound immunoglobulin. Reconstruction of macrophages-depleted, B-cell enriched or B-cell depleted cultures with autologous macrophages resulted in an increase of the transfromation efficiency up to the values of non-fractionated leukocyte preparations. Addition of heterologous human embryonic lung fibroblasts resulted in a similar increase. The results support the interpretation that EBV transforms only those cells of the hematopoetic system which are derived from the bone-marrow entity. The transformation efficiency is considerably increased by co-cultivation of lymphocytes with macrophages and heterologous human fibroblasts which seem to excert a feeder-layer effect by enhancing survival of lymphocytes in vitro.