lmmunotoxins were constructed by attaching native ricin A chain, ricin A, chain and recombinant r i c h A chain to the mouse monoclonal IgG,, antibody Fib75 by means of a disulphide linkage using the hetero-bifunctional cross-linker SPDP. The Fib75 immunotoxins were of similar composition and exerted identical cytotoxic effects against the EJ human bladder carcinoma cell line in tissue culture. All 3 immunotoxins broke down to the same extent upon incubation with glutathione in vitro. The clearance of the immunotoxins from the circulation of normal Wistar rats was determined following i.v. administration. The concentration of intact immunotoxin in serum samples taken at various intervals up to 48hr after injection was measured by a ricin A chain-specific ELISA. The Fib75 immunotoxin made with native ricin A chain was removed from the circulation most rapidly. Fib75-recombinant rich A chain persisted in the circulation at a higher level than Fib75-ricin A, chain, A higher proportion of the ricin A, chain immunotoxin was lost from the bloodstream during the a-phase. The P-phase half-lives of Fib75-recombinant ricin A chain and Fib75-ricin A, chain were similar, consistent with the identical susceptibility of the immunotoxins to cleavage by glutathione. The presence of the complex-type oligosaccharide side-chain on the A, chain may have accelerated the clearance of the A, chain immunotoxin in relation to that of the immunotoxin made with the aglycosyl recombinant A chain.