Specimens infused with or suspended in a mixture of 10-30% poly(vinylpyrrolidone) and 2.07-1.61 M sucrose can often be more easily frozen-sectioned than those infused with sucrose alone. The pH of such a mixture can be efficiently adjusted to neutrality by using Na2CO3. Use of poly(vinylpyrrolidone) causes little or no increase in the background level of immunolabelling. Adsorption staining of ultrathin frozen sections with a mixture of uranyl acetate and poly(vinyl alcohol), i.e. a simple thin-embedding of the sections in such a mixture, produces positive staining effects that are often enough to delineate structures of many organelles. When OsO4-treated frozen sections are stained with uranyl acetate and further adsorption-stained with a mixture of lead citrate and poly(vinyl alcohol), the overall staining effects are similar to those observed in double-stained conventional sections.