Quercetin (QC) (5, 7, 3', 4' -tetra oxyflavonolol) is an ubiquitous flavonoid in many plants. The influence of QC on the growth of B16 melanotic melanoma in C57BL/6 mice and activity of some acid hydrolases in the tumor homogenates were investigated. Two series of experiments were carried out: In the first experimental group mice were inoculated s.c. with 10(6) of tumor cells (TC) suspended in 1 ml of saline. TC were obtained from the current serial passages. In the second series of experimental group mice were inoculated with melanoma cells preincubated 15 min. in different concentrations of QC. Mice of both series were divided into three subgroups. Mice of the first series were treated with QC i.p. every second day in a dose of 0.1 mg, 0.5 mg or 1.0 mg (total dose of 1.0 mg, 5.0 mg or 10.0 mg per mice). Animals of the second series did not obtain any treatment. After the nineteenth day of experiment the mice were killed, tumors excised and weighed. Tumor tissue pieces were homogenized for enzyme activity determination. Fragments of tumor tissue were taken for electron microscopy (EM) investigation. In mice injected i.p. with QC mean tumor weight was significantly higher than in control I. The mean tumor weight in the first experimental group was higher than in control from 170% to 196% and in the second experimental group from 69% to 147%. Enzymes activity was also higher in both experimental groups as compared to controls. Arylsulphatase activity in the first group was higher from 102% to 144% and in the second one - from 97% to 115% than in control I. Acid phosphatase activity was higher from 100% to 155% in the first experimental group and from 56% to 161% in the second one. Cathepsin D activity was greater from 133% to 333% and from 113% to 300%, respectively. EM studies revealed the presence of greater number of Golgi structures and primary lysosomes in experimental groups of tumors (mice treated with QC and mice with melanoma preincubated in QC). These results clearly indicate that QC significantly enhances melanotic melanoma growth and increases acid phosphatase and cathepsin D activity in these tumors. The mechanism of QC action on the melanotic melanoma is not fully understood and remains to be defined.