Fig. 1. EGF accumulation in the bulk medium in the presence and absence of EGFR-blocking antibodies. Monolayers of cells were induced to produce EGF at a rate of approximately 1000 moleculesxcell^-1xmin^-1 using tetracycline. Fresh medium with () or without () 5.0 µg/ml 225 mAb was added to the cells and, at the indicated time, the medium was collected and assayed for EGF using an ELISA. The fraction of EGF captured, as calculated from the ratio of EGF concentrations without and without 225 mAb, remained constant over 12 hours (insert). The error bars represent the s.d. of one experiment in duplicate.