Tetrahymena pyriformis Wh 14 was incubated with HCB (0.001 - 0.5 ppm) in Erlenmeyer flasks under continuous stirring at 30degreesC for 10 days in an incubator. HCB decreased growth as deduced from measurements of dry weight, carbohydrates, and total nitrogen. Total nitrogen and carbohydrates are decreased drastically while dry matter is reduced only by a factor of two. The excretion of a compound into the nutrient solution which yields a colour with modified Ehrlich reagent like porphobilinogen is very strongly increased by HCB. All effects observed were very well correlated with the concentration of this compound applied.