Malignant tumours are characterised by genetic changes which can, in the majority of cases, be detected using cytogenetic methods. Cytogenetic research of great number of tumours indicate that karyotypic changes are not allocated in the genome by chance, and that they are commonly tumour-specific clone disorders. Pathogenetically, the most important step is to identify so-called primary clone disorders which are, in the majority of cases, single. The object of this thesis was to discover cytogenetic characteristics of reoccurring disorders in small cell populations in the multiple myeloma and malignant lymphoma. Cytogenetic characteristics were detected using conventional technique of striping chromosomes and FISH technique in the level of interphase nucleus and/or miotic chromosomes in 43 cases diagnosed with multiple myeloma and in 80 cases diagnosed with malignant lymphoma. An original scientific contribution in identifying rare chromosome abnormalities which is used for a more specific classification of malignant lymphoma and multiple myeloma is achieved. The research will contribute to a better clinical approach in diagnosis, prognosis and treatment of these tumors.