The morphology and Ag-staining of nucleoli in human multiple myeloma cell lines RPMI 8226 and U 266, distinguished from each other in the differentiation degree, were quantitatively studied, and the production of immunoglobulins or their fragments by the line cells was evaluated throughout 7 days after cell seeding. The less differentiated cell line RPMI 8226 and the high differentiated cell line U 266 were revealed to differ in both the initial level of immunoglobulin production and dynamics of immunoglobulin accumulation in culture medium. The total number of Ag-stained nucleolar-organizer regions (AgNORs) per nucleus in cells RPMI 8226 was significantly higher than in cells U 266 in all times after seeding of the cells. In both cell lines changes in the quantity and shape of nucleoli and also in the total number of AgNORs per nucleus and pattern of AgNORs distribution within nucleoli correlated with the cell cycle phase. Relationships between morphofunctional changes in nucleoli and the differentiation degree and proliferative activity of the cells, and also between the number of Ag-positive nucleolar-organizing metaphase chromosomes and the functional activity of interphase AgNORs are discussed.