Treatment of type I cryoglobulinemic glomerulonephritis depends on identifying the monoclonal clone responsible for cryoglobulin secretion. We report the case of a woman in whom the culprit clone proved exceptionally difficult to detect, making treatment guidance highly uncertain. Using the RACE RepSeq technique combined with innovative bone marrow cell sorting, and later through direct visualization within the kidney, we identified a likely plasma cell clone responsible for the cryoglobulin. Despite targeted interventions, the disease remained refractory to conventional therapies, teclistamab, a bispecific T-cell–engaging antibody redirecting T cells against plasma cells, achieved efficacy.