Bystander immune suppression has been demonstrated in experimental models of oral immune tolerance induction. This phenomenon is associated with expression of transforming growth factor (TGF)-beta1 and T-helper cell (Th) 2 cytokines. We have studied serum levels of Th cytokines and B- and T-lymphocyte subsets in chronic idiopathic thrombocytopenic purpura (ITP), a disorder in which the production of platelet autoantibodies might be caused by a cytokine network dysregulation. Forty-six patients with ITP were separated into three groups depending on the platelet count (pltc): (1) < 50 x 10(9)/l, (2) 50-150 x 10(9)/l and (3) > 150 x 10(9)/l. We found significantly elevated plasma levels of the Th3 cytokine TGF-beta1 in patients with pltc >150x10(9)/l (23.5+/-2.8ng/ml), compared with patients with pltc <50x10(9)/l (2.3+/-0.6 ng/ml; P<0.0001), patients with pltc 50-150x 10(9)/l (7.2+/-1.7 ng/ml; P<0.0001) and healthy volunteers (9.8+/-1.3 ng/ml; P<0.01). The serum levels of the Thl cytokines interleukin (IL)-2 and interferon (IFN)-y were below the detection limits of the assays. Likewise, the Th2 cytokine IL-4 was not detectable or was very low both in patients and controls. The serum levels of IL-10, a Th2 cytokine, were within the assay range and patients with pltc <50 x 10(9)/l had significantly lower levels (0.6+/-0.1 pg/ml) than both patients with pltc 50-150 x 10(9)/l (1.8 +/- 0.1 pg/ml; P<0.005) and healthy volunteers (1.4+/-0.1 pg/ml; P<0.005). Furthermore, patients with pltc <50 x 10(9)/l and splenectomised patients had significantly higher levels of CD4 + CD25 + activated T cells [26.2 +/- 14.8% (P<0.05) and 26.7+/-11.9% (P<0.005), respectively] than healthy controls (16.5+/-4.0%). Also, the number of natural killer (NK) cells among patients with pltc >150 x 10(9)/l were significantly elevated (26.6+/-16.0%; P<0.05) compared with controls (17.4+/-7.6%). In conclusion, our data corroborate previous findings of elevated numbers of activated T cells in chronic ITP patients with active disease, but neither a clear-cut Th1 nor a Th2 serum cytokine profile could be established. However, ITP in remission was associated with elevated TGF-beta1, which might be a part of a bystander immune suppression. We propose that the effect of possible expression of TGF-beta1 by oral immune tolerance induction deserves to be explored in ITP patients with an active disease.