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.