The relative importance of four interrelated possibilities for cyanide removal in aerated microbial systems was investigated. In an aerated system at neutral pH with both cyanide and glucose present, the relative importance for cyanide acclimated heterogeneous cultures appears to be? 1) stripping; 2) biological metabolism? 3) adsorption onto biological floe, and? 4) chemical reaction in solution with substrate. At higher pH, chemical reactions in solution become significant. The chemical reaction of cyanide with substrate was investigated in sealed glass ampoules using glucose as substrate and inorganic buffers. The reaction was found to be pseudo-first order and pH dependent, with an optimum pH near 11.. The cyanide-glucose reaction products were found to be biodegradable by both acclimated and unacclimated heterogeneous cultures in shake flask and BOD bottle systems. Adsorption properties were investigated using sealed, stirred glass reactors containing bacteria and potassium cyanide in water buffered at pH 7. with inorganic buffers. Very little adsorption was evidenced after one hour in reactors containing a starved nonflocculating pure culture of Bacillus megaterium.ATCC Strain 23. However, up to 15 percent adsorption occurred in systems containing a stirred flocculent heterogeneous culture. Thus, biological adsorption effects are relatively unimportant as a cyanide removal mechanism. Stripping effects were investigated using starved heterogeneous bacteria in an aerated microfermenter at neutral pH. Cyanide and carbon dioxide in the off-gas were trapped in sodium hydroxide solution, separated, and analyzed. Stripping removed up to 8 percent of original cyanide, and tests using K14CN revealed that a small amount of cyanide was metabolized. Cyanide metabolism was studied using starved, acclimated heterogeneous cultures in an aerated microfermenter containing glucose as substrate. Tests using K14CN showed that up to 5 percent of the cyanide was metabolized as evidenced by 14CO2 production. Bacillus pumilus and Bacillus mecraterium did not aero bically metabolize significant quantities of glucose, but both species metabolized tryptic soy broth in the presence of up to 1 mg/1 cyanide (as CN).