Early diagnosis of leukemia is crucial in the recovery of people suffering from this serious disease. This is why numerous automatic methods are being developed for the detection of this disease. This paper studied microscopic images from the ALL-IDB database. These are pictures of microscopic blood samples from people suffering from acute lymphocytic leukemia. The procedure of digital image processing for extracting lymphocyte nuclei from the scientific work "Diagnosis of Leukemia and its types Using Digital Image Processing Techniques" in the literature marked under [2] was studied. The methods of digital image processing mentioned in the mentioned paper were applied, and reference values of the roundness of immature and mature lymphocytes, i.e. their nuclei, were calculated for the images from the ALL-IDB2 database. It was observed that according to the roundness values it is not possible to completely determine whether it is an abnormal lymphocyte. For this reason, additional detection criteria were introduced, such as values of the are of lymphocyte nuclei and the ratio of the total number of lymphocytes and erythrocytes on the microscopic image. Incomplete segmentation of lymphocytes was also observed in some images, and these values were ignored by an additional criterion in the detection itself. The detection procedure successfully detected leukemia in 66.67% of cases.