Monoclonal antibodies specific for three T cell alloantigens linked to the immunoglobulin complex on chromosome 12 were used to establish the order of expression of these antigens on immunocompetent cells and in ontogeny. Modification of monoclonal antibodies with fluorescein isothiocyanate (FITC) and use with anti-FITC and complement has amplified lytic capacity of the monoclonals and allowed us to complete a distribution study of Tsud, Tindd, And Tthyd alloantigens on immunocompetent cells. Tthyd is expressed on both cortisone-sensitive and cortisone-resistant thymocytes; Tsud and Tindd are on "mature" cortisone-resistant cells. Tthyd is also expressed on a Thy-1.2-bearing recirculating marrow cell but is undetectable in the peripheral T cell pool. In contrast, resting spleen and lymph node T cells express Tsud and Tindd; antigen-activated populations express these two cells in high frequencies. These antigens must be markers for relatively differentiated cells because "nude" animals, which have pre-T cells, fail to express these determinants. All three antigens segregate independently in our T cell hybrids, arising from adult peripheral node cells, supporting the hypothesis that these are three separate structural products of a gene complex. In contrast, fetal T cell hybrids fail to express these antigens. The appearance of all three antigens on the cell surface in ontogeny is postnatal; Tthyd is expressed at days 1-2, Tindd at days 2-3, and Tsud at days 5-6. If the T cell isotype genes are organized similar to the immunoglobulin loci, then the parallels in maturational expression on immunocompetent cells and in ontogeny would lead one to speculate a gene order of Tthyd leads to Tindd leads to Tsud. Orientation with respect to the centromere is unknown.