OBJECTIVE: To assess the bioactivity of circulating prolactin (PRL) in serum samples from patients with systemic lupus erythematosus (SLE) using 2 novel homologous in vitro bioassays, and to correlate PRL bioactivity with lupus activity. METHODS: Serum samples from 98 SLE patients with and without disease activity were tested for immunoreactive and bioactive concentrations of PRL. RESULTS: Patients with active disease exhibited higher bioactive serum PRL levels in homologous bioassays (p