The purpose of this study was to evaluate the effect of β-(1,3/1,6)-D: -glucan isolated from Pleurotus ostreatus (β-glucan-PO) on prophylactic treatment of adjuvant arthritis (AA) with methotrexate (MTX) in rats. Groups of rats with AA were treated with methotrexate (1 mg/kg/week), β-glucan-PO (1 mg/kg every second day) or their combination for the period of 28 days from adjuvant application. Body mass, hind paw swelling, arthrogram scores and a level of serum albumin were measured as markers of inflammation and arthritis. Treatment with low dose of MTX significantly inhibited the markers of both inflammation and arthritis. MTX and its combination with β-glucan-PO significantly increased body mass of arthritic rats. β-glucan-PO administered alone significantly decreased both the hind paw swelling and arthritic score. In combination with MTX, β-glucan-PO markedly potentiated the beneficial effects of MTX, which resulted in a more significant reduction of hind paw swelling and arthritic scores. The concentration of albumin in the serum of arthritic controls was significantly lower than in healthy controls. Both MTX alone and the combination treatment with MTX + β-glucan-PO significantly inhibited the decrease in serum albumin. β-Glucan-PO increased the treatment efficacy of basal treatment of AA with MTX.