T here are multiple immune defects in T cells from recipients after bone marrow transplantation (BMT). This study examines recipient T cells for increases in intracellular ionized calcium concentration [( Ca2+]i) after binding the T cell receptor-CD3 complex with anti-CD3 MAb. PBL from 10 of 23 short-term recipients (less than 1 yr after BMT) responded poorly (less than 35% of control) to anti-CD3 stimulation and PBL from 9 of 23 had blunted calcium flux responses (35-70% of control). Purified CD2+, CD56- cells from seven additional short-term recipients including three autologous marrow recipients were closely examined, and a sizable proportion of CD3+ cells from six of seven recipients did not increase [Ca2+]i after anti-CD3 stimulation. The decreased magnitude of the responses was due to decreased numbers of responding cells and not to a decrease in mean CD3 fluorescent intensity or in calcium flux responses on a single cell basis. Five of seven long-term recipients (greater than 1 yr after BMT) had PBL that responded normally and two of seven had PBL with blunted calcium flux responses. The data show that the signal transduction response mediated by the CD3-antigen receptor as measured by calcium flux is defective early after autologus or allogeneic BMT.