Analysis of the B cell repertoire is complicated by the huge diversity inherent in the germ line determined combinatory. Making use of knockout technology, K-deficient mice have been obtained. They constitute a shrewd model to follow the expression of an Ig minilocus, such as theλone, in the normal condition compared with classical transgenic models. Indeed, in contrast to wild type mice, in which only 5% ofλB cells are produced, these mutant mice exclusively produceλpositive B cells. Although, theλlocus is well characterized and has a relatively simple organization, the mechanistic and selective pressures that govern its utilization are still poorly understood. The analysis of theλB cell repertoire in K-deficient mice, should therefore bring more conclusive informations. Here we present theλsubtype distribution in the various cellular compartments of the K-deficient mice, and discuss the rules that can be responsible for this distribution. Our recent data indicate that theλsubtype proportions in the bone marrow and the spleen result, for the major part, from mechanistic processes (i.e., recombinase accessibility, production of V-J functional joint and H/L pairings) while theλproportions found in the peritoneal cavity ensue from selective processes. Finally, the capacity to respond to various antigens is discussed from such a generatedλB cell repertoire.