The yield and assembly of an IgG1 antibody and its derived F_ab fragment were compared inNicotiana andArabidopsis. The results obtained showed a lot of interclonal variability. For 45% of the primary transgenic calluses, antigen-binding entities represented less than 0.1% of the total soluble protein (TSP). Only two of the 103 analysed transformants contained more than 1% of antigen-binding protein, with 1.26% being the highest yield. Analogous amounts of complete antibody and F_ab accumulated in primary callus tissue. Moreover, yields were in the same range for both species as far as primary callus tissue is concerned. However, the accumulation of the F_ab fragment in leaf tissue of regenerated plants differed significantly betweenNicotiana andArabidopsis. The F_ab fragment accumulated to only 0.044% of TSP inNicotiana leaves but up to 1.3% inArabidopsis leaves. Furthermore, both species showed differences in the assembly pattern of the complete antibody. WhereasArabidopsis contained primarily fully assembled antibodies of 150 kDa,Nicotiana showed an abundance of fragments in the 50 kDa range.