SUMMARY: An osmotic barrier for phosphate very near the visible surface of the chick-embryo cell appears to regulate the reciprocal exchange of inorganic orthophosphate between the medium and a component of the acid-soluble inorganic phosphate of cells kept in monolayer culture. Some non-reciprocal transfer of inorganic phosphate occurs, which may or may not be due to cell damage, and the apparent phosphate-impermeable volume decreases by half after 3 hr. contact with phosphate at 0°G. At 2°G the exchange is at least 97 % inhibited. The phosphateimpermeable volume after cell rupture is less than 10 % of the intact-cell value, when internal phosphate is released in a form shown by mild separation to be mostly inorganic orthophosphate. Cells lose internal phosphate more slowly in absence of external phosphate, and the addition of external phosphate increases the loss rate to a maximum at physiological concentrations and higher. Such a barrier provides information on the nature of the cell surface, and shows that adsorption, penetration and release of vesicular stomatitis virus occurs without any detectable damage to the surface controlling the phosphate-exchange rate.


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