By using a sensitive ELISA with rhNPM as the antigen, we showed that anti-NPM antibodies are present in most male WB lupus-prone mice and are therefore a constant feature of the antibody response in these animals: anti-NPM positivity appeared early in life, increased with age, and peaked at three to four months, before death. This evidence concerns the gene NPM1 and systemic lupus erythematosus.