Abstract: Cryostat sections of biopsy specimens from psoriatic lesions before and after methotrexate (MTX) treatment were labelled in situ by a series of monoclonal antibodies including anti-T cell subsets antibodies. The results showed that in the stationary stage of psoriasis, there may be a chronic inflammatory process and the progressive stage of psoriasis may be related to the abnormality of T4/T8 ratio. MTX by mouth may exert anti-inflammatory and immuno-regulating actions on the disease.