Sliding frictional forces between spherical silica particles (diameter, 5 mum) and silicon wafers were measured with a technique related to the colloidal probe technique. The variation in friction force with applied normal force is found to follow the variation in contact area as predicted by the JKR theory (K. L. Johnson, K. Kendall, A. D. Roberts, Proc. R. Soc. London A 324 301, 1971). This supports the hypothesis that for a single microasperity contact the frictional shear strength is constant.