Whatman black and green mixed ester membranes are composed of cellulose acetate (20%) and cellulose nitrate (80%). These membranes are characterized by a smoother and more uniform surface than pure nitrocellulose filters. Also, the color contrast provided by the filter surface facilitates particle detection and minimizes eye fatigue. Compared to pure nitrocellulose filters, mixed ester membranes have a smoother and more uniform surface as well as offer color contrasting, making it easier to detect particles. Green membranes are particularly useful when the particles under analysis are black in color. In microbiological colony counting procedures, the color contrast between the surface and the colonies facilitates the counting process. Whatman black and green mixed ester membranes are available plain for automatic colony counting applications as well as gridded to assist in manual counting procedures. Black and green membranes provide contrast between residue or cell colors and the filter without having to counterstain the membrane.