This work describes a new method of evaluating washing effectiveness based on analysis of light scattering on washed fabric. A relationship was found for determining the maximal degree of cleaning achieved using given washing agents in given physico-chemical conditions. The errors in the method for evaluating the degree of cleaning are also discussed. A method of evaluating washing effectiveness was verified, based on the analysis of the effect of the number of washings on the degree of cleaning. Four commercially available washing agents were tested. The method proved useful for evaluating the degree of cleaning of cotton fabrics and for classifying washing agents according to their washing power. -- AATA