Lymphoproliferative activities produced by cloned thymic myoid cell 871207B were analysed by immunological and biochemical methods. The lymphoproliferative activities were separated into two fractions by DEAE-Sepharose CL-6B chromatography: one is in the fraction passed through the column and the other in the fraction eluated from the column with a low concentration of NaCl. The eluated fraction induced the proliferation of interleukin-1 (IL-1)-dependent D10N4 M cells. This activity was abrogated by an anti-IL-1 alpha antibody, but not an anti-IL-1 beta antibody. Expression of IL-1 alpha mRNA was also detected in 871207B cells. The thymocyte proliferative activity found in the fraction passed through the DEAE-Sepharose column was further separated into three fractions by heparin-Sepharose column chromatography: (1) the fraction passed through the column, (2) the fraction weakly bound to the column, and (3) the fraction firmly bound to the heparin column. The fraction passed through the heparin column sustained the growth of IL-6-dependent MH60.BSF-2 cells. IL-6-specific mRNA was found in 871207B cells. The thymocyte proliferative activity of the fraction firmly bound to the heparin column was neutralized with an anti-IL-7 antibody. The biological activity of the fraction weakly bound to the column remained to be elucidated. These results suggest that thymic myoid cells produce IL-1 alpha, IL-6, IL-7 and unidentified lympho-stimulatory factors, all of which play significant roles in many steps of T-cell development in the thymus.