SummaryDerivatives of the protein ferritin, differing in their net charge, were presented to cultures of peritoneal macrophages, HeLa, KB and L cells. Unmodified ferritin was not taken up by the continuons line cells. However, as the negative charge carried by the inulecule was reduced, a point was reached when these cells would take up the test protein. The continuous line cells bchaved as a homogeneous population in the sense that either all or none of the cells in the culture would take up a particular protein. The macrophage culture did not behave in this way. A small proportion of the macrophage population would pinocytose the untreated ferritin and as the negative charge carried by the molecule was decreased the proportion of cells taking up the protein increased markedly. The penetration of protein into KB cells was only observed when the test protein would bind to the cell surface. The adsorbed protein fixes to the eell surface in a patchy manner and does not form a uniform layer over the whole surface. The pattern of adsorption suggtests that there are well defined receptor areas on the cell surface where binding can occur.