Incubation of unprimed splenocytes directly with synthetic peptides induces CD8+ primary CTL in vitro. The ability of peptide-pretreated LPS blasts or H-2Db-expressing Drosophila melanogaster cells to induce peptide-reactive primary CTL was analysed using synthetic peptides corresponding to distinct and overlapping amino acids in SV40 T antigen Db-restricted T-cell epitopes I, II-III, and V. Peptide-pretreated, but not untreated, LPS blasts induced strong peptide-reactive CTL in splenocytes of naive C57 BL/6 mice. The reactivity of these CTL was indistinguishable from that of CTL induced by direct stimulation of spleen cells with peptides. In contrast, effector cells induced by untreated or peptide-pretreated drosophila cells expressed equivalent cytolysis both against untreated and peptide-pretreated syngeneic target cells. They also strongly lysed the allogeneic (H-2d) P815 target cells. The cytolytic activity was expressed by CD8+ T lymphocytes. However, the lack of specificity and restriction was probably due to stimulation of LAK cells as well. The CTL-inducing potential of the drosophila cells was stable at 46 degrees C. Peptide-activated splenocytes cultured for 1 month in medium supplemented with spent medium from drosophila growth culture were 75% B cells and 25% CD4+ T cells. Taken together, these results indicated that LPS blasts are efficient APC for induction of primary CTL and that drosophila cell-derived molecule(s) are capable of inducing broad-reactive effector cells and may also assume the function of leukocyte growth factor(s).