The degree of microviscosity (eta), and lipid fluidity (LFU) of cellular membranes of normal and leukemic lymphocytes obtained from peripheral blood and bone marrow of normal donors and acute lymphatic leukemic (ALL) patients was quantitatively monitored by fluorescence polarization analysis with the aid of the fluorescent lipophilic probe 1,6-diphenyl-1,3,5-hexatriene when embedded in cellular membranes of intact cells. The results have shown a marked decrease in eta and a significant increase in LFU in lymphocytes obtained from both peripheral blood and bone marrow of ALL patients at admission when compared to both T- and B-lymphocytes obtained from peripheral blood of normal donors. Moreover, both dynamic parameters, eta and LFU, show normal characteristic values in lymphocytes obtained from bone marrow of ALL patients in complete hematological remission. Since in few cases a decrease in eta and an increase in LFU were observed in bone marrow lymphocytes isolated from ALL patients in remission, the possibility that these dynamic parameters may serve as a diagnostic tool for an early detection of a new relapse is discussed.