SummaryIn the current study, thein vitroviability and resistance to methylprednisolone (MP) was investigated in lymphocytes from patients with cystic fibrosis (CF) and normal controls. The addition of MP to phytohemagglutinin-stimulated lymphocyte cultures inhibits the proliferative response in a dose-dependent way. The dose response curves to MP were similar in CF cultures and controls (P > 0.05). Neither preincubation nor pulse exposure to MP revealed any difference between normal and CF lymphocytes.Skin fibroblasts from patients with CF have been reported to show increased resistance to the toxic effect of dexamethasone (1–3, 5–6). This study was undertaken to investigate the suppressive effect of glucocorticoids on another cell culture system using MP on phytohemagglutinin-stimulated lymphocytes.