The capacity of liposomes constituted by dycetyl-phosphate (0.009 mM), cholesterol (0.017 mM), lecithin (0.003 mM), and myristic (0.1 mM), stearic (0.1 mM), or oleic acid (0.1 mM) to modify the lymphocyte response to Brucella melitensis antigens in mice was studied. Mice treated with antigens mixed with liposomes containing myristic, stearic or oleic acid had higher antibody titres than mice given antigen suspended in a saline solution. Liposomes alone, without Brucella antigens, resulted in increased 3H-thymidine incorporation by lymphocytes both in vivo and in vitro. The addition of polyclonal activators (LPS and ConA) caused a further increase of 3H-thymidine uptake. Moreover, spleen lymphocytes from mice inoculated with Brucella antigens mixed with the liposomes had a significantly lower population of B lymphocytes (10%), and a notable increase in the Tc lymphocytes (20%). Autoradiography of sections of popliteal ganglia of treated mice showed that the radioactivity was concentrated mainly in the membrane structures of the cell.