Milk producers in the UK are paid according to their production of milk fat, protein and lactose. The production quota system also includes an element restricting fat production. It is therefore important to be able to predict the changes in production of milk solids arising from changes in feed inputs. There are few published predictive models for this purpose (e.g. Sporndly 1989, Emery, 1978) and none under UK conditions. This is a preliminary study to identify and quantify some of the factors affecting yields of milk constituents as a first stage in the development of predictive models.