Relative transduction efficiency with retroviral vector-producing clones was assayed by cocultivating TF-1, a human CD34+ hematopoietic cell line and YR-2, a sheep B-lymphoid cell line, with LacZ containing vector-producing cells, and then by scoring the percentage of X-Gal+ cells. At the same time, viral titer was estimated by titration assay with murine fibroblasts. Results clearly demonstrated a lack of correlation between viral titer and efficiency of transduction into hematopoietic cells, which depends neither on the type of packaging cell line, PG-13 and GP-envAM12 in this study, nor on the type of LacZ containing retroviral vector. These results strongly favor consideration of interactions between producers and target cells of the study for the screening of producing cell lines.