VIP fails to suppress PMA/ionomycin-stimulated T-cell induction of TNF-α, IL-10, and IFN-γ production. Fresh, elutriated human T lymphocytes were plated out at 1 × 105 cells per well in a U-bottomed 96-well plate and pretreated with VIP (a,b,c), or dibutyryl cAMP (d,e,f) for 1 hour prior to stimulation with 50 ng/ml PMA and 0.5 μg/ml ionomycin and incubated for 24 hours at 37°C in a 5% CO2 humidified atmosphere, after which time supernatants were harvested and assayed for IL-10 (a,d), TNF-α (b,e), and IFN-γ (c,f) by ELISA. Data are mean cytokine levels in pg/ml of triplicate culture supernatants ± SD, showing a representative of n = 4 replicate experiments. *P < 0.05; **P < 0.01. PMA, phorbol 12-myristate 13-acetate; TNF-α, tumour necrosis factor α; VIP, vasoactive intestinal peptide.