Macropinocytosis has emerged as an important nutrient supply pathway that 
sustains cell growth of cancer cells within the nutrient-poor tumor 
microenvironment. By internalizing extracellular fluid through this bulk 
endocytic pathway, albumin is supplied to the cancer cells, which, after 
degradation, serves as an amino acid source to meet the high nutrient demands of 
these highly proliferating cells. Here, we describe a streamlined protocol for 
visualization and quantitation of macropinosomes in adherent cancer cells grown 
in vitro. The determination of the "macropinocytic index" provides a tool for 
measuring the extent to which this internalization pathway is utilized within 
the cancer cells and allows for comparison between different cell lines and 
treatments. The protocol provided herein has been optimized for reproducibility 
and is readily adaptable to multiple conditions and settings.