The characteristics of sulphur colloids have been investigated using colloids labelled with 35S as well as with 99mTc. The results support a model in which technetium is incorporated in the body of the particle rather than on its surface. Elemental sulphur on the surface of the particle is susceptible to attack both by sulphide ions and by protein and other materials containing --SH groups. The ready conversion of sulphur to soluble polysulphides means that sulphur colloid particles undergo conspicuous changes in diameter, both while standing in a closed vial, and on injection into body fluids. This behaviour casts doubts on the value of routine particle-size measurement as a quality control procedure.