In the presence of serum-containing medium, BHK cells attached and spread during a 1-h period onto a 3-5 nm thick serum layer absorbed on the substratum surface. The closest approach of the plasma membrane to the serum layer was observed to be about 9nm, which was determined by tilting the sectioned cells in a goniometer holder. Bundles of microfilaments or other cytoplasmic specializations were not observed in association with the regions of close contact. However, in the space between the plasma membrane and the adsorbed serum layer, a diffusely stained material could be visualized after fixation/staining by the tannic acid-glutaraldehyde technique. This technique also permitted increased clarity of visualization of trilaminar appearance of the plasma membrane. The distribution and mobility of anionic sites on the surfaces of attached and spreading cells was determined by labeling with polycationic ferritin. We observed movement of polycationic ferritin into large clusters on the cell surface, collapse of cell surface microextensions, and endocytosis, all of which were similar to our previous findings utilizing cells in suspension. However, the absolute amount of ferritin bound to the upper cell surface was less than that previously observed when suspended cells were put under similar labeling conditions. Also, polycationic ferritin did not appear to penetrate between the lower cell surface and the substratum.

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