We have compared the accumulation of recombinant variable heavy-chain portions [VHH (variable heavy-chain antibody from camelids)] of camelid antibodies in a variety of subcellular compartments produced in planta. The VHH coding sequences were optimized for expression in thale cress (Arabidopsis thaliana) and placed individually or as fused tandem heterodimers in synthetic plant-organelle-targeting cassettes designed to target the protein to either the cytoplasm, ER (endoplasmic reticulum), protein storage vacuole or chloroplast. Accumulation of individual VHHs was only detected in plants transformed with the ER-targeting cassette, whereas accumulation of the tandem VHHs was detected for all cassettes and was the highest with the ER cassette [0.1-0.7% (w/w) of total soluble proteins]. The ability of the plant-produced tandem VHH to reduce TNFalpha (tumour necrosis factor alpha) cytotoxicity was found to be comparable with previously characterized recombinant VHHs. In vitro antigen binding and functional stability in rumen fluid were determined on both prokaryotically expressed and plant-expressed tandem VHHs. The plant-produced VHH did not appear to be any more stable in rumen fluid than other soluble plant proteins; however, it was able to bind equally well to the antigen in the presence or absence of rumen fluid.