Newly-hatched chickens, isogenic for the B locus, were treated with high doses of cyclophosphamide, either alone or in combination with surgical bursectomy. The cyclophosphamide treatment initially caused virtual absence of bursal lymphoid cells and, later, complete destruction of the normal bursal architecture. It also caused an initial decrease in the lymphoid population of the thymus. However, thymic morphology was completely restored in chickens that were 15 days old or older. The most striking features in the morphology of the spleen and of the other peripheral lymphoid tissues of cyclophosphamide-treated birds was the absence of germinal centers and of the plasma cell line. No clear morphological differences could be detected between birds that were treated with cyclophosphamide alone and those subjected to cyclophosphamide treatment in combination with surgical bursectomy. The immunological capacities of normal, cyclophosphamide-treated and cyclophosphamide-treated bursectomized chickens were evaluated, starting with 1-month-old birds. The experimental groups of animals lacked or were profoundly deficient in agglutinating antibody to B. abortus antigen and to sheep erythrocytes after primary and secondary stimulation, while the normal controls responded well. Profound deficiency of IgM and IgG, as measured by the radial diffusion technique, was also obtained in the majority of the birds treated with cyclophosphamide alone or in combination with surgical bursectomy in the newly-hatched period. No clear differences could be detected, in the lack of capacity to respond to antigenic stimulation or to form immunoglobulins, between the birds that were treated with cyclophosphamide alone and those treated with cyclophosphamide together with bursectomy. Cellular immunological functions of normal and of cyclophosphamide-treated chickens were evaluated with regard to capacity to reject allografts and to elicit the graft-versus-host reaction. Cyclophosphamide treatment in the newly-hatched period did not affect the cellular immunological capacities of 1-month-old or older birds. Thus, by using this experimental protocol, cyclophosphamide can be used to accomplish ‘chemical bursectomy’, resulting in a permanent, severe deficiency of the humoral immunological capacities more frequently than can be obtained with other presently available bursectomy methods, while leaving the cellular immunological capacities intact.