Lymphoid leukosis (LL) was successfully controlled in a commercial basic breeding line of White Plymouth Rock chickens. The control method has been developed for breeder flocks and consists of three elements:-In the flock under study, homogenates of embryos from all eggs collected during a number of I4‐day periods are tested for the presence of LL viruses.-Only eggs from hens that have been shown not to shed virus in their eggs are used for the production of progeny. The offspring are reared in isolation during the first two months of life, at which time the age‐related resistance against tumour formation by LL viruses appears to be sufficiently developed.-The chickens are subsequently inoculated intramuscularly with LL viruses of subgroups A and B transferred to a conventional chicken house.