CARD11 is expressed in lymphocytes and functions as a scaffold for the recruitment of signalling partners MALT1 and BCL10 into a protein complex, which is essential for relaying signals from the antigen receptor to NFκB and AP-1/cJUN transcription factors. Gain-of-function somatic mutations in CARD11 are found in approximately 10% of the aggressive ABC DLBCL subtype of lymphoma. Recently, patients with polyclonal B cell lymphocytosis-like disease were found to have germline gain-of-function mutations. They all presented with splenomegaly and B cell lymphocytosis in infancy, with dramatically elevated levels of IgM+IgD+CD19+CD20+ circulating B cells. Here, we characterise an ENU-induced germline Card11 mutant mouse strain as a model for investigating human B cell diseases. Card11M365K mice have increased circulating IgM+IgD+ B cells, which are hyper-responsive to anti-IgM stimulation as they grow larger in size and express more NFκB-responsive markers such as CD25. CARD11M365K enhances proliferation of B cells induced by anti-IgM stimulation and prevents self-antigen induced B cell death. However, unlike patients with germline CARD11 mutations, the Card11M365K mice do not develop splenomegaly and B cell lymphocytosis.