A cell fusion technique was used to produce hybridomas between the T lymphoma cell line, EL-4, derived from C57BL (H-2(b)), and an enriched population of human gamma globulin (HGG)-specific suppressor T cells prepared from the spleens of HGG-tolerant CBA mice (H-2(k)). Membrane fluorescence analysis of the hybridoma cells within 6 wk of cell fusion revealed expression of H-2(k) and I-J(k) gene products as well as H-2(b) antigens. Sonicates prepared from hybridomas which contained I-J(k) cells were tested for suppressive activity in vivo in irradiated mice given HGG-primed cells, dinitrophenyl (DNP)-primed cells, HGG-DNP, and horse erythrocytes. Among 18 such hybridoma lines, 6 showed specific suppressive activity, 5 nonspecific suppression, and 7 no suppression. Most lines progressively lost, with time, those properties derived from the normal parent cell. By about 3 mo after fusion few cells expressed CBA markers and only one cell line (number 77) retained some specific suppressive activity. In parallel with the losses was an alteration in chromosome number from near-tetraploid, soon after cell fusion, to near- diploid. Preliminary results with the T lymphoma-sensitive hypoxanthine aminopterin thymidine cell line, L5178, indicate retention of the expression of surface markers derived from the normal parent for 18 wk after hybidization. This suggests that T lymphoma cell lines may have to be screened for their capacity to produce hybridomas with stable properties.