Streptavidin was conjugated to the second-generation TAG-72 monoclonal antibody CC49 at lysine amino acids, oxidized carbohydrates or reduced disulfides on the immunoglobulin. The Streptavidin immunoconjugates were radiolabelled with 111In-DTPA-biocytin and their immunological characteristics evaluated in vitro and in vivo. FPLC analysis showed a single peak (mol. wt > 350 kDa) for the lysine conjugate and sulfhydryl conjugate (mol. wt ~ 210 kDa) but multiple peaks (~ 210 to > 350 kDa) for the carbohydrate conjugate. There were only small differences in immunoreactivity against bovine submaxillary mucin in vitro. However, in mice bearing subcutaneous LS174T tumours, the lysine conjugate exhibited significantly lower tumour uptake (< 2% i.d./g) compared to the other streptavidin-CC49 conjugates (10-40% i.d./g) or DTPA-CC49 (14-18% i.d./g). Due to the monomeric nature and smaller molecular size of the sulphhydryl conjugate, and its similar in vitro and in vivo characteristics compared with DTPA-CC49, this conjugate has been selected for future pretargeting studies with 111In and 90Y biotin.