To create antibody molecules with improved functional properties, a growth factor (insulin-like growth factor 1, IGF1) was used to replace the constant region of a chimeric mouse-human IgG3 anti-dansyl antibody. The chimeric heavy chain was expressed with an anti-dansyl-specific chimeric kappa light chain. The IgG3-IGF1 chimeric protein retained its specificity for the antigen dansyl. The chimeric proteins bound to the IGF1 receptors of the human lymphoblast IM-9, albeit with reduced affinity, and elicited some of the same biologic effects (increased glucose and amino acid uptake) in human KB cells as did human IGF1, but with reduced specific activity. The reduced affinity and biologic activity may result from several things: the presence of the unprocessed IGF1 moiety, the large size of the IgG3-IGF1 chimeric protein (160 kDa) compared with IGF1 (7 kDa), and three amino acid substitutions in rat IGF1 compared with human IGF1, which may lead to decreased affinity for the human IGF1 receptor. The chimeric proteins show that it is feasible to produce a new family of immunotherapeutic molecules targeted to growth factor receptors.