Turk's solution is a staining solution used in hematology to count leukocytes (white blood cells). The solution consists of a dilute dye called gentian violet (also known as crystal violet or methyl violet) and a dilute solution of acetic acid.

Türk's solution serves two main purposes: first, it stains the leukocyte nuclei to make them visible under the microscope, and second, it lyses the erythrocytes (red blood cells), giving a clearer picture of the leukocytes in the slide. Acetic acid is responsible for lysing the erythrocytes, while gentian violet stains the leukocyte nuclei.

To determine the leukocyte count, a blood sample is mixed with Türk's solution and then spread on a counting chamber slide. The leukocytes can then be counted under a microscope, with their number serving as an indicator of the presence of infection, inflammation, or other immunologic disease.