The transfer of Chernobyl Cs-134 and Cs-137 to milk in dairy cows from a diet, predominated by silage, was studied under normal farming conditions. The study was carried out in the period December 1986 till August 1987 and comprised three periods with different levels of radioactive contamination of the silage. Transfer coefficients for both radionuclides were derived from measured radiocaesium concentrations in the diet components and in mixed milk samples. Average silage intake was calculated on basis of total need which depends on milk production. The transfer coefficients calculated for Cs-134 and Cs-137 after a 6 weeks feeding period of high-level post-Chernobyl silage were 0.26 and 0.27% d/l, respectively. These results indicate that a transfer coefficient of 1.2% d/l, from tracer and fallout studies and recommended for predictive purposes, is a conservative estimate in view of the post-Chernobyl experience. The transfer of caesium isotopes from silage to milk in dairy cows is dynamically described with a simple two-compartment model of Pelletier and Voilleque. The most important choices of parameter values to be made in matching calculated and measured concentrations in milk, are those for F and u/m. F is the fraction of ingested activity that is excreted in milk plus urine, and u/m is the ratio between the transfer rates to urine and milk, respectively. The study was carried out with financial support from the Dutch Ministry of Agriculture and Fishery.