The document is a proceedings of the fourth symposium on the 'Application of Short-Term Bioassays in the Analysis of Complex Environmental Mixtures' held March 27-29, 1985 in Chapel Hill, NC. The early chapters of the volume are devoted to new bioassay techniques that are directly applicable to the monitoring of environments contaminated with genotoxic chemicals. Microbiological methods have been further refined to meet the special needs of atmospheric monitoring so that very small samples may now be efficiently tested. New in situ methods utilizing green plants actually avoid many of the usual difficulties of sample collection and preparation and offer special advantages in monitoring wastewater, sludges, and hazardous wastes. Increased emphasis has been placed on a comprehensive assessment of the potential of complex mixtures to cause various kinds of genetic damage. Studies comparing in vitro and in vivo data on a coal conversion byproduct, on polycyclic aromatic hydrocarbons, and on mineral fibers are reported in separate papers. Later chapters are devoted to investigations on the fractionation and biological evaluation of specific chemical components within complex mixtures. Techniques have been used to demonstrate the formation of potent mutagens, the nitropyrenes, in environmental mixtures as well as in foods. The effect of atmospheric transformation of automotive exhaust organics and of woodstove emissions were reviewed. Chamber studies showed that atmospheric processes can both increase and decrease the mutagenicity of emitted organic compounds.