In order to detect the possible contamination with bacterial endotoxins of some preparations of Transfer Factor, Limulus amebocyte lysate test was performed. The results obtained (TF preparations positive with L.T. for endotoxin assay) call sur detection to evaluate very carefully the immunologic responses in vitro ad in vivo, with these biological products contaminate can induce. In fact, bacterial endotoxins with their multiple potent effect on biologic system, represent a contant possible surce of variability in experimental biology and pathology.