Here we report that human B lymphocytes can be positively selected directly from buffy coats applying the anti-CD19 antibody AB1 coupled to magnetic beads. This isolation protocol is highly efficient and the isolated cell population is of very high purity and viability. As judged by cell cycle analysis and various parameters for cell activation, the cells are still in a resting state after isolation. Furthermore, different functional assays have shown that the isolation procedure does not interfere with either activation or proliferation/differentiation of CD 19 selected cells as compared to negatively isolated cells. As a consequence of cross-linking during the isolation process, the CD 19 antigen is temporarily down-regulated as measured by AB1 binding. Despite this decreased expression, monoclonal antibodies to the CD19 antigen nevertheless inhibited anti-μ plus B cell growth factor induced B cell activation as reported also for negatively isolated cells. Taken together, the presented data strongly suggest that B cells isolated through the CD 19 antigen can be used in critical functional assays.