The duration of detectable colostral antibodies to the glycoprotein antigen of bovine leukemia virus was studied in calves which were born to bovine leukemia virus-infected cows, but showed no serologic evidence of prenatal infection. Colostral antibodies detectable by an agar-gel immunodiffusion test (AGIT) persisted for < 1 mo. to 6 mo. (mean 2.9 mo.) in the 139 calves examined. Colostral antibodies were detectable 1-5 mo. longer by radioimmunoprecipitation assay than by the AGIT in 22 of the 24 calves studied comparatively. The mean duration of colostral antibodies in those 24 claves was 3.8 mo. (min-max, 2-6 mo.) for the AGIT and 6.0 mo. (min-max, 4-9 mo.) for the radioimmunoprecipitation assay.