B-cell development. In the BM of mice from all experiments only 10% to 20% of huCD19⁺ cells expressed cell surface IgM. In the spleen 60% to 75% of CD19⁺ cells also expressed IgM. PB from all mice revealed that 75% to 90% of huCD19⁺ cells also expressed IgM. This was true for mice transplanted with input CD34⁺ cell fractions known to be contaminated with low levels of CD19⁺ cells (experiments 4 and 7, Table 1), as well as for mice transplanted with CD34⁺ input fractions devoid of any CD19⁺ cells (experiments 5 and 8, Table 1).