Immunotoxins synthesized with the pan-T-cell monoclonal antibody T101 and ricin, acetylricin, or ricin A-chain have been compared. Native ricin was acetylated with N-acetylimidazole to block the galactose-binding site of the toxin B (binding)-chain. In the presence of lactose, both whole-ricin-containing immunotoxins were selectively cytotoxic but the ricin A-chain conjugate was less effective in blocking cellular protein synthesis. Immunotoxin-treated cells cultured in fresh growth medium exhibited no growth, declining viabilities, and no protein synthesis activity. Lymphocytes treated with T101:ricin or ricin did not form clusters or colonies when plated in 0.3% Bacto-agar. Ammonium chloride markedly enhanced the efficacy of T101:ricin and T101:ricin A-chain. Our results suggest that: (a) all immunotoxins were selectively cytotoxic; (b) in the presence of ammonium chloride the effectiveness of the T101:ricin A-chain conjugate approached that of T101:ricin; and (c) the toxin B-chain may facilitate conjugate internalization and/or processing.