Guinea pig lymphotoxin (LT) produced by stimulation of sensitized lymph node cells, specifically by antigen or non-specifically by concanavalin A, was studied by methods previously used to characterize guinea pig migration-inhibitory factor. The results showed that LT induced by either antigen or mitogen was heat labile, eluted from Sephadex columns with peak activity in fractions containing molecules of 35,000 to 55,000 m.w., had an electrophoretic mobility at pH 9.1 similar to albumin and a buoyant density on CsCl similar to protein. It was destroyed by chymotrypsin but not by neuraminidase. These studies show that guinea pig LT can be readily differentiated from guinea pig migration-inhibitory factor. On the other hand, the LT produced by antigen stimulation has many features in common with LT produced by stimulation with concanavalin A.