The manual gives revised procedures for Level 1 environmental assessment biological tests, and supersedes the first edition, EPA-600/7-77-043 (NTIS No. PB 268484), published in April 1977. The revised biological procedures complement the Level 1 chemical and physical procedures published in October 1978 as IERL-RTP Procedures Manual: Level 1 Environmental Assessment (Second Edition), EPA-600/7-78-201 (NTIS No. PB 293795). Level 1 is a screening phase that identifies, categorizes, and ranks the pollutant potential of influent and effluent streams from industrial and energy-producing processes. The manual is a guide to sampling and analysis professionals in planning and executing the bioassay portion of a phased environmental source assessment program. The manual gives the goals, strategies, and philosophy of a phased approach to environmental assessment. It introduces collection and pretest handling procedures for environmental samples and the recommended Level 1 biological test protocols used to analyze the samples. Basic quality control procedures are discussed, as are possible bioassay procedures for Levels 2 and 3.