The aim of this work was to study vitamin B 1 2 retention during manufacture of six fermented dairy products. Careful validation of a commercial radio protein-binding kit showed this assay to be suitable after optimisation of sample pre-treatment and control of the kit for possible matrix effects. In fermented milks, vitamin B 1 2 concentrations decreased by 40-60%, compared with the starting milk, during storage of the final product at 4 o C for 14 days, most likely attributed to consumption by starter cultures. In cottage cheese, hard cheeses and blue cheese, 18-56% of the vitamin B 1 2 originally present in the milk was retained. Removal of the whey fraction was the dominant factor reducing vitamin B 1 2 retention in cheeses, while the fermentation by starter cultures hardly affected vitamin B 1 2 concentrations.