Psoriasis is a T-cell mediated inflammatory skin disease which has been associated with group A, beta-haemolytic streptococcal infections. Four 20 a.a. long M6-peptides sharing 5-6 a.a. sequences with human epidermal keratins were identified. To investigate the role of potentially cross-reactive T cells in the pathogenesis of psoriasis, interferon-gamma (IFN-gamma) and interleukin-4 (IL-4) responses of circulating T cells to these peptides were analysed by ELISPOT and RT-PCR in 14 psoriatic patients, 12 healthy individuals and six patients with atopic dermatitis (AD). Untreated psoriatic patients' responses were significantly higher to these peptides than healthy and AD controls, while responses to a control M6-peptide, not sharing sequences with keratin, were negligible in all groups. No difference was found in response to streptokinase/streptodornase (SK/SD). M6-protein and peptides exclusively elicited IFN-gamma production, with little IL-4 production, even in AD patients. Interferon-gamma responses to all the M6-peptides were abolished after successful treatment of psoriatic patients, but responses to SK/SD were unaffected. The results indicate that active psoriasis is associated with Th1-like cells responding to streptococcal M6-peptides sharing sequences with human epidermal keratin. This is consistent with the hypothesis that psoriasis may be induced and exacerbated in susceptible individuals by M-protein specific Th1-like cells that cross-react with human epidermal keratin.