Problems concerning the use of "markers" in cell fractionation experiments are currently in the forefront in many areas of cellular investigation. Points of view and standards of application vary widely. This report has emerged from a debate during the 6th International Subcellular Methodology Forum, Guildford, U.K., 25-28 July 1978. Marker criteria are discussed, first in a general sense and then in terms of specific cell components. Use of multiple marker criteria is stressed along with selective modulation, cytochemistry, and morphological analyses for fraction identification and evaluations of yield and purity. Finally, some specific recommendations are made toward documentation that journal editors might expect from authors.