Two different lymphotoxins synthesized by human blood lymphocytes stimulated with phytohemagglutin (PHA) are described. The two toxins are called α-LT and β-LT relative to their elution order on gel chromatography. Their m.w. are 75,000 and 45,000 daltons, respectively. Both toxins appear as early as 7 hr after the addition of PHA, with the amount of β-toxin exceeding that of α-LT initially. Both toxins are differentiated from a third toxin (adherent cell toxin, ACT) made by plastic-adherent cells without requiring mitogen-stimulation. Depletion of macrophages or B cells does not affect the synthesis of either lymphotoxin. Monospecific antisera were elicited to α-LT. Antisera elicited to β-LT also neutralized α-LT but to a significantly lesser degree. α-LT appears to be the lymphotoxin described by most other workers. β-LT is unstable at 37°C which may explain why the low m.w. lymphotoxin has not been described previously.