This Test Guideline describes six methods that permit the screening of chemicals for ready biodegradability in an aerobic aqueous medium. The methods are: the DOC Die-Away, the CO2 Evolution (Modified Sturm Test), the MITI (I) (Ministry of International Trade and Industry, Japan), the Closed Bottle, the Modified OECD Screening and the Manometric Respirometry.A solution, or suspension, of the test substance, well determined/described, in a mineral medium is inoculated and incubated under aerobic conditions in the dark or in diffuse light. The running parallel blanks with inoculum but without test substance permits to determined the endogenous activity of the inoculum. A reference compound (aniline, sodium acetate or sodium benzoate) is run in parallel to check the operation of the procedures. Normally, the test lasts for 28 days. At least two flasks or vessels containing the test substance plus inoculum, and at least two flasks or vessels containing inoculum only should be used; single vessels are sufficient for the reference compound. In general, degradation is followed by the determination of parameters such as DOC, CO2 production and oxygen uptake. The pass levels for ready biodegradability are 70% removal of DOC and 60% of ThOD or ThCO2 production for respirometric methods. These pass values have to be reached in a 10-d window within the 28-d period of the test.