Surface of IgM-expressing lymphocytes infected by bovine leukemia virus and with/without persistent lymphocytosis (BLV+PL-, BLV+PL+) in cattle is heterogenous. Three distinct morphological structures of B lymphocytes were found in these groups by immuno-scanning electron microscopy (ISEM). B lymphocytes with generally elongated and pleomorphic microvilli and sometimes short ruffles were observed in infected, and as well as in non-infected animals. A second morphological type endowed with a relatively small number of elongated villi-like veils, and a third morphological type characterized by a relatively small number of short microvilli or blunt stub-like microvilli covering the smooth surface were seen, mainly in BLV+PL+ and seldom in BLV+PL- Cells of the second type were more intensively labeled, than lymphocytes of the third type.