Surface-expressed immunoglobulin (Ig) has been shown to have a critical role in allelic exclusion of Ig heavy (H) and light (L) chains. Although various degrees of suppression of endogenous Ig expression are observed in Ig transgenic (Tg) mice, it was not clear whether this difference is due to different onsets of Tg expression or to different levels of Tg expression, which are obviously affected by integration sites of the transgene. In this study we generated antierythrocyte antibody Tg mice that carry tandem joined H and L chain transgenes (H+L) and confirmed that homozygosity of the transgene loci enhances the level of transgene expression as compared with heterozygosity. Suppression of endogenous H and L chain gene expression was stronger in homozygous than in heterozygous Tg mice. Similar results were obtained in control Tg mice carrying the H chain only. These results suggest that there is a threshold of the B cell receptor expression level that induces allelic exclusion. In addition, despite the same B cell receptor specificity, the size of Tg autoreactive B-1 cell compartment in the peritoneal cavity is larger in homozygous than in heterozygous mice, although the number of the Tg B-2 cell subset decreased in the spleen and bone marrow of homozygous Tg mice as compared with heterozygous Tg mice. By contrast, homozygosity of the H chain alone Tg line, which does not recognize self-antigens, did not increase the size of the peritoneal B-1 subset. These results suggest that the size of the B-1 cell subset in the Tg mice may depend on strength of signals through B cell receptors triggered by self-antigens.