Cardiotoxin D from Naja naja siamensis is cytotoxic to T-lymphocytes above 150 femtomoles/10(6) cells. Equivalent lysis of macrophages or B-lymphocytes requires at least 1000 times more toxin. Reduction and carboxamidomethylation of cardiotoxin D does not effect T cell lysis. At higher toxin concentrations, 50% T-cell lysis occurs within 10 min. Splenocytes cultured with mitogens are up to five times more susceptible to toxin than unstimulated cells. Cardiotoxin D may directly disrupt the plasma membrane, since lysis is unaltered at 4 degrees C.