Biological medical means for treating psoriasis are disclosed. The invention relates to the use of L-serine (Ser) and L-isoleucine (Ile) for the manufacture of a pharmaceutical composition for treating or preventing psoriasis. The invention further relates to a pharmaceutical composition comprising said amino acids as active ingredients. Preferably the composition further comprises at least one essential trace element selected from the group consisting of chromium (Cr), tin (Sn), selenium (Se), vanadium (V), wolfram (W) and zinc (Zn), and folic acid.
