The ability of cells to internalize through the clathrin pathway was determined by observing transferrin uptake by confocal microscopy. Cells were serum starved for 0.5 h prior to the addition of Alexa-568 labelled transferrin (25 mg/ml) in GMEM at 37uC for the times indicated in the results. For quantification of transferrin uptake, cells with cytoplasmic labelling for Alexa-568 transferrin were scored as positive. Alexa-568 labelled dextran (5 mg/ml) was added to the cells as described for transferrin. In all experiments, .98% of the control cells showed transferrin or dextran uptake.