Forty-six patients with venous immunologically mediated or lymphoproliferative diseases received goat antihuman thymocyte antisera globulin (ATG) therapy. Heterophile antibody production occurred in 91 % of these patients, following ATG administration. The heterophile antibody was absorbable with both guinea pig kidney and boiled beef erythrocyte antigens. This corresponds to a form of heterophile antibody previously reported as a "serum sickness" variety. The heterophile antibody was composed of a mixture of gamma G and gamma M antibodies and was partially absorbed by ATG. Serum sickness did not occur in any of these patients, in spite of the production of heterophile antibodies. It is recommended that the involved heterogenetic antigen be termed "foreign serum" variety, and the resulting antibody described as a "foreign serum" type of heterophile antibody.