Cytotoxic antibody—toxin conjugates made using antibodies and ribosome-inactivating proteins (RIPs) are prepared using chemical crosslinking methods (1 ,2 and Chapter 13). Gel permeation chromatography is used as a first step to purify conjugate molecules from the reaction mixture. This procedure removes protein aggregates, the excess of RIP employed in the conjugation reaction, and low molecular weight byproducts. However, a significant fraction of the resulting conjugate preparation consists of unconjugated antibody that cannot be completely separated from the conjugate on the basis of size discrimination alone (see Chapter 13).