The rate of increase and decay of radio contamination secreted in sheep's milk, resulting from a constant level of radiocesium in the animals' diet, was investigated. Ten lactating ewes were used in the experiment. For a period of 12 d the animals fed on contaminated grass, resulting in a daily radiocesium intake of 832 Bq per animal. They were subsequently returned to a contamination-free diet and were monitored for another 9 d. Throughout the period of the experiment, 134Cs and 137Cs concentrations in the animals' milk were measured daily with an 18% efficiency, high-resolution Ge detector. The data were in satisfactory agreement with the predictions of a simple two-compartment theory. The transfer coefficient, describing the steady-state equilibrium in this model, was measured as fm = 0.058 +/- 0.007 dL-1.