Chick embryos were bursectomized at 5 days of incubation according to a novel surgical technique described in this article. This method yields birds that are able to hatch and are devoid of the physiologic deficiencies resulting from the previously used method, which involved resection of the cloacal and posterior embryonic region. The bursectomized embryos were grafted in situ with a quail bursa of the same age, which thereafter became chimeric through chick host hemopoietic cell invasion. By means of species-specific antibodies, the chimeric condition revealed 1) that the bursal epithelium expresses a unique antigenic determinant (MB1 determinant), until now considered to be an exclusive feature of blood vessel endothelium and hemopoietic cells, and 2) that this determinant appears in bursal epithelium at the time and site of hemopoietic cell invasion. The other point arising from this work concerns the apparent constitutive Ia expression by perifollicular blood capillary endothelial cells in normal and chimeric bursas.