The detergency of cotton clothes soiled with clay particles was determined in water, linear alkylben-zene sulfonate (LAS), sodium tripolyphosphate (STPP) and their mixtures. The surface reflectance has been measured on the basis of the Kubelka-Munk equation. A discussion is made on the detergency from the view point of the interaction of the clay particles and cotton fibers. The zeta-potentials of the clay particles in different washing solutions were measured based on a laser Doppler effect. The particle to particle Hamaker constant (A22/3) and fiber to fiber Hamaker constant (A11/3) in washing solutions were calculated from contact angles of water to particle and fiber respectively. The particle to fiber Hamaker constant (A12/3) in washing solution was calculated from fiber Hamaker constant (A11), particle Hamaker constant (A22) and washing solution Hamaker constant (A33) . No significant correlationship between the-se Hamker constants and the detergency has been recognized. On the other hand, the higher the zeta-potential of clay particle in washing solution, the greater the detergency. A fairly good correspondence has been observed between the maximum value of total potential curves, obtained from the potentials of at-traction and repulsion, and the detergency.

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