Penicillin G (PNG) has been demonstrated to elicit T-cell responsiveness in vitro in allergic patients by means of a lymphocyte transformation test (LTT). As it was not clear how, or in what form, the stimulatory PNG determinants were inducing cellular proliferation, we compared the immune response elicited by different PNG preparations. Peripheral blood lymphocytes (PBL) of patients with proven PNG allergy were isolated, and proliferative responsiveness to soluble PNG alone, soluble PNG-protein conjugates (BPO-HSA, BPO-PL, BPO-HEX), and non-reactive penicilloate salts, was evaluated. An autologous mixed lymphocyte culture (MLC) using penicilloylated stimulator cells, was used to test responsiveness to membrane-bound PNG. We found that the addition of either 1000 micrograms/ml of potentially-reactive PNG to cell cultures, or of penicilloylated autologous cells was stimulatory, whereas non-reactive PNG salt, and soluble PNG conjugates were not stimulatory. Considering current and earlier findings, it appears that T cell immunity in these patients is directed towards PNG-modified "self", as PNG-modified autologous cells are potent stimulators in PNG-allergic individuals.