The Measurement of the Extracellular Space in an In Vitro System of Human Leucocytes

1. The extracellular space of an in vitro system of human leucocytes in an artificial extracellular fluid has been measured with Na₂³⁵SO₄, ¹³¹I-labelled human serum albumin and ¹²⁵I-labelled polyvinylpyrrolidone of high molecular weight.

2. There was no significant difference between the space measured with Na₂³⁵SO₄ and ¹²⁵I-labelled polyvinylpyrrolidone. The space measured with ¹³¹I-labelled human serum albumin was significantly larger than that measured with ¹²⁵I-labelled polyvinylpyrrolidone.

3. High-molecular-weight ¹²⁵I-labelled polyvinylpyrrolidone appears to be a useful extracellular marker for this in vitro system.