BACKGROUND It was the aim of this study to evaluate the effects of the well-known immunosuppressive drugs ciclosporin A (CsA), tacrolimus and sirolimus on the intracytoplasmic cytokine expression of neonatal immune cells. METHODS Immunosuppressive drugs were added to whole blood cultures of neonatal cord blood samples (n = 17) and peripheral blood samples of adults (n = 17) in vitro prior to stimulation of lymphocytes with phorbol 12-myristate 13-acetate (PMA)/ionomycin or monocytes. RESULTS Upon exposure to ciclosporin A (500 ng/mL) or tacrolimus (25 ng/mL) the number of cytokine expressing T cells was almost completely blocked in neonatal T cells while sirolimus (10 ng/mL) only inhibited intracytoplasmatic tumour necrosis factor alpha (TNF-alpha) expression (mean% positive cells; 4.0 +/- 2.1% vs. 1.09 +/- 0.6%, p = 0.003), but mildly stimulated the intracellular expression of interleukin (IL)-2 (24.4 +/- 6.5% vs. 28.1 +/- 7.1%, p = 0.041). In cord blood lymphocytes, the inhibitory effect of ciclosporin A and tacrolimus was dose-dependent (e.g. IL-2: control, 12.3 +/- 5.33%, ciclosporin A 5 ng/mL, 10.1 +/- 5.5%; 50 ng/mL, 7.1 +/- 4.7%; 500 ng/mL, 1.2 +/- 0.3%; tacrolimus 0.25 ng/mL, 9.3 +/- 4.9%; 2.5 ng/mL, 6.1 +/- 3.3%; 25 ng/mL, 1.0 +/- 0.6%), while the function of adult lymphocytes was only impaired at high doses of both compounds. In contrast, the number of cytokine expressing monocytes was not influenced by ciclosporin A and tacrolimus except for a minor decrease of TNF-alpha producing neonatal monocytes after addition of tacrolimus (17.9% vs. 13.9%, p = 0.031). Interestingly, sirolimus was shown to inhibit intracellular IL-6 production in adults (63.1 +/- 12.7% vs. 52.0 +/- 16.0%, p = 0.005), but in neonatal monocytes intracellular IL-6 expression was stimulated (53.5 +/- 22.0% vs. 64.7 +/- 19.1%, p = 0.041). CONCLUSIONS The potent dose-dependent inhibitory effect of ciclosporin A and tacrolimus in cord blood lymphocytes provides the basis for further studies on functional immaturity of the neonatal immune system and for future strategies to optimize umbilical cord blood transplantion. Sirolimus was demonstrated to have a distinct effect on neonatal immune cells as shown by increased expression of IL-2 in lymphocytes and IL-6 in monocytes, while only lymphocytic TNF-alpha expression was inhibited.