In the present investigation the chemical effect on colour fastness of Phyllanthus muellerianus dyed wool was studied. Samples were assessed in terms of the colour fading to spotting with dilute hydrochloric acid, with 56% acetic acid, steeping for two minutes at room temperature in 28% ammonium hydroxide without rinsing, exposed to concentrated ammonia fume for 24 hours and spotting with 10% sodium carbonate and calcium hydroxide respectively. Contact with dilute hydrochloric acid brings about significant changes in the color and fastness properties of natural dyed wool yarn. The colours however, remain stable after exposure or contact with alkalis and 56% acetic acid. It can be concluded that Phyllanthus muellerianus dyed wool fabric have good colour fastness to perspiration and except with mineral acids.