A colorectal tumour-directed immunotoxin, ICI D0490, has been constructed by linking recombinant ricin A-chain to C242, a mouse monoclonal antibody, by means of a methyl-hindered disulphide bond. Recombinant ricin A-chain and a hindered disulphide linker were anticipated to confer favourable pharmacokinetic properties on the immunotoxin. The pharmacokinetics of ICI D0490 have been studied in mice following single and repeated i.v. administration. The concentrations of intact immunotoxin in mouse plasma at various time intervals after injection for up to 96 h were measured by a solid-phase enzyme-linked immunosorbent assay (ELISA) and the data analysed by both model-dependent (two compartment) and model-independent methods. Following a single i.v. bolus dose of 2.5 mg kg-1 (50% of the LD10 in mice), the clearance of ICI D0490 from the plasma was extremely slow; 34 microliters min-1 kg-1, t1/2 beta = 33 h. Model-dependent and model-independent analyses gave comparable results with steady state volumes of distribution of 93 and 69 ml kg-1, respectively. The two compartment analysis gave an initial volume of distribution (63 ml kg-1) which is consistent with the predicted plasma volume. Over the dose range 0.05-5 mg ICI D0490 kg-1, plasma levels at 2 and 24 h were linearly related to dose (r > or = 0.98) indicating that at doses up to 5 mg ICI D0490 kg-1 clearance does not appear to have a saturable component. Repeated doses of ICI D0490 (1 mg kg-1 day x 5) did not lead to drug accumulation. These studies demonstrate that ICI D0490 has excellent in vivo stability and persistence which, in conjunction with activity and toxicity data, identify ICI D0490 as a promising candidate for clinical evaluation in the treatment of colorectal cancer.