Bioconcentration of pollutants by aquatic microorganisms has been of interest to environmental scientists for several years because of the position of these organisms in the food chain. Also, similarities between accumulation by microbial species and higher organisms are of interest from a mechanistic viewpoint. Thus, an exhaustive review of the literature has been conducted and the state of knowledge critically evaluated with respect to the kinetics, mechanisms, and magnitude of accumulation. Particular emphasis in the evaluation is directed toward demonstrating that most available data are consistent with a simple mechanism that is 'passive' rather than active in the biological sense. Further, it is shown that reliable measurements of the extent of accumulation must take into account both time and biomass concentration. Methods currently in use for experimental measurement and data analysis are discussed to show that many common limitations can be avoided. Some of the more important aspects to be considered involve the use of labeled materials, washed cells, Freundlich isotherm, and molecular structure. In addition, data are presented to show that the octanal-water partition coefficient can be used to predict the potential extent of accumulation.