Vitamin B-12 was measured in samples of normal ovine colostrum and mil
k and in sera of lambs and ewes taken at various stages of lactation,
from lambing up to 16 d. Colostrum obtained within 24 h of lambing con
tained high concentrations of vitamin B-12, but within a few days the
levels fell. The lambs received high levels of vitamin B-12 from their
dams' colostrum but a parallel decrease of vitamin B-12 levels in lam
bs' serum and ewes' milk was subsequently observed. Milk vitamin B-12
was analysed by the microbiological method (Lactobacillus leichmannii,
ATCC 7830) and a radioisotope dilution (RID) technique. Milk vitamin
B-12 concentrations were greater with the microbiological technique co
mpared with the RID method. The radioassay method has been shown to be
as reliable as the microbiological method for measuring vitamin B-12
in ovine milk except for very low levels.