AbstractThe capacity of bone marrow‐derived surface immunoglobulin‐positive (sIg+) human and mouse immature B cells, generated eitherin vitroorin vivo, to change their light (L) chain expression, has been assayed by the number of cells which changein vitrofrom one type of L chain to the other type, or to no sIg at all. Immature sIg+B cells were generatedin vitrofrom sIg−precursor cells from human or mouse bone marrow. The immature sIg+cells expressed RAG‐1. Human sIg+cells expressed xfr; and λ L chains in ratios between 1:1 and 3:1, whereas in mouse cells, this ratio ranged from 10:1 to 20:1. Upon reculture of the human and mouse xfr;+sIg+cells, about half of them remained xfr;+, a quarter became λ+, and another quarter became sIg−. Between 1 and 3 expressed both xfr; and λ chains. Of the human λ+cells, about two‐thirds remained λ+, only 1 to 2 became xfr;+, while the other third became sIg−. Again, between 1 and 3 expressed both xfr; and λ L chains. These results indicate that expression of sIgM in the B cell membrane does not terminate L chain gene rearrangement, and that some order exists in xfr; versus λ gene rearrangements. Hence, human and mouse xfr;+immature B cells can become λ+, but very few of the λ+cells can become xfr;+, and both can become sIg−. Further, human CD10+/sIg+xfr;+and λ+cells and mouse B220low/sIglowxfr;+cells enriched from bone marrow,i.e.immature B cells differentiatedin vivo, changed their Ig phenotype uponin vitroculture, but in lower frequencies. By contrast, human and mouse mature B cells did not change their L chain or Ig phenotype. Hence, at least a part of the sIg+immature B cells in bone marrow retain the capacity to change their L chain and Ig phenotype, and this capacity is lost when they become m
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