SLP-65-/- pre-B cells show a high proliferation rate in vitro. We have shown previously that lambda5 expression and consequently a conventional pre-B cell receptor (pre-BCR) are essential for this proliferation. Here, we show that pre-B cells express a novel receptor complex that contains a micro heavy chain (microHC) but lacks any surrogate (SL) or conventional light chain (LC). This SL-deficient pre-BCR (SL-pre-BCR) requires Ig-alpha for expression on the cell surface. Anti-micro treatment of pre-B cells expressing the SL-pre-BCR induces tyrosine phosphorylation of substrate proteins and a strong calcium (Ca2+) release. Further, the expression of the SL-pre-BCR is associated with a high differentiation rate toward kappaLC-positive cells. Given that B cell development is only partially blocked and allelic exclusion is unaffected in SL-deficient mice, we propose that the SL-pre-BCR is involved in these processes and therefore shares important functions with the conventional pre-BCR.