Multiple myeloma is often associated with humoral immunodepression in both man and mouse. When mice bearing the humorally immunodepressive plasmacytomas TEPC-183 and SPQC-11 were injected with SRBC, the rise of serum haemolysins was significantly less than that of non-tumour-bearing mice. Mice with the plasmacytomas MPC-11 and MOPC-315 have an antibody response similar to normal mice when injected with SRBC. Following immunization, normal mice and those bearing MPC-11 showed a 2- to 3-fold increase in total spleen lymphocytes. Mice bearing TEPC-183 or SPQC-11, the plasmacytomas causing an impaired antibody response, has significant increase in spleen lymphocytes under the same conditions. Mice bearing MOPC-315 had a very high initial count of spleen lymphocytes, which did not further increase upon immune stimulation.Incubation of lymphocytes from plasmacytoma-bearing mice with PHA did not produce an increase in TdR incorporation and in some cases even caused a decrease in TdR incorporation.Lymphocytes from mice bearing TEPC-183, SPQC-11, and MOPC-315 incorporated less TdR in response to LPS than did normal mice. On the other hand, mice bearing MPC-11 incorporated about as much TdR as did normal mice following LPS stimulation. Thus, the defect in the ability to respond to LPS in vitro correlated with the lack of an increase of spleen lymphocytes in mice bearing these tumours following antigenic stimulation in vivo.No immunodepressive properties of serum from mice with plasmacytoma could be detected.