Treatment of chickens as embryos and in the first six days after hatching with mg amounts of anti-TCR2 (alpha beta T cell antigen receptor) antibody plus thymectomy 6 to 8 days after hatching suppressed by more than 90% the TCR2 and TCR1 (TCR gamma delta) subpopulations in the blood. Thymectomy alone resulted only in a profound reduction (greater than 90%) of the absolute number of TCR1 cells, while the absolute number of TCR2 cells was reduced only by 65%. We were interested in the regulation of the immunoglobulin synthesis in these T cell deficient chickens. We determined serum IgG, IgM and IgA levels and found a profound selective reduction (by about 90% on average) of the serum IgA level in anti-TCR2 antibody treated plus thymectomized chickens. In chickens which had been thymectomized only, the serum IgA level was reduced on average only by about 60%. Serum IgG and IgM levels were not reduced in any group. Thus, there was a positive correlation of the serum IgA level and the absolute number of TCR2 cells in all groups. These results indicate that T cells which play a role in IgA specific isotype switching or in IgA B cell differentiation belong to the TCR2 subpopulation.