The toxicity of chlorinated benzenes was assessed using bacteria. Chlorobenzenes studied were monochlorobenzene (108907), o-dichlorobenzene (95501), m-dichlorobenzene (541731), p-dichlorobenzene (106467), 1,2,3-trichlorobenzene (87616) (TCB 1), 1,2,4-trichlorobenzene (120821) (TCB 2), 1,3,5-trichlorobenzene (108703) (TCB 3), 1,2,3,4-tetrachlorobenzene (634662) (TCB 4), 1,2,3,5-tetrachlorobenzene (634902) (TCB 5), 1,2,4,5-tetrachlorobenzene (95943) (TCB 6), and pentachlorobenzene (608935) (PCB). Concentrations of 20 to 1000 milligrams per liter (mg/l) were incubated in a growth medium containing bacillus, isolated from activated sludge, and resazurin solution. Cultures were assayed for microbial dehydrogenase activity. All chlorinated benzenes stimulated activity by 4.9 to 11.0 percent at the 20mg/l concentration. At 100mg/l, all compounds stimulated activity except TCB 1 and 2, which inhibited by 74.6 and 51.4 percent, respectively. PCB, TCB 3, 4, and 5 stimulated activity at all concentrations. The other compounds began inhibiting activity at concentrations between 100 and 500mg/l. The authors conclude that chlorinated benzenes tend to bioconcentrate in the phospholipid fractions of cell membranes where most dehydrogenases are located. The biological activity of a specific toxicant depends on its molecular geometry and electronic properties.