In this study, the effect of toxaphene (camphechlor) on ATPase activity in the microsomal fraction of the Unio tumidus's digestive gland was determined. Toxaphene is a man-made mixture consisting of polychlorinated monoterpens, predominantly bornanes. This compound was primarily used as an insecticide, but in 1982 was officially banned because of its destructive effects on human and animal health. Toxaphene can be transported in the air at long distances and can persist in air, soil and water for years revealing acute and chronic toxicity towards aquatic organisms and wildlife, the increasing risk of cancer in both humans and animals. The microsomal fraction isolated from digestive glands was exposed to 1 x 10(-3) M, 1 x 10(-5) M and 1 x 10(-7) M of toxaphene. The obtained data showed that toxaphene induced a loss of ATPase activity in all used concentrations. The Lineweaver-Burk plots for microsomal Na+K+-ATPase in the presence or the absence of toxaphene as an inhibitor indicated a competitive type of inhibition.