A large proportion of intestinal intraepithelial lymphocytes (IEL) comprises alpha beta and gamma delta T-cell receptor (TcR)-bearing T cells. The numbers of alpha beta and gamma delta IEL are reported to be very different between germ-free and conventional microbial conditions. In this study, we investigated the kinetics of both types of TcR-bearing cells after microbial colonization in germ-free mice and the influence of thymus deprivation on IEL populations during the microbial association process. Immediately after association with microbes in germ-free animals, the number of alpha beta TcR-bearing IEL gradually increased. Fourteen days after microbial association the number of alpha beta IEL equalled that of gamma delta TcR-bearing IEL. Approximately 1 month after microbial association, the number of alpha beta IEL was several times greater than that of gamma delta IEL, having almost reached the level in conventional mice reared in a conventional animal room after birth. On the other hand the number of gamma delta IEL hardly changed throughout this microbial association process. Two-colour analysis involving anti-alpha beta TcR and anti-Lyt-2 or Lyt-3 antibodies showed that the major fraction of IEL that increased after microbial association comprised alpha beta TcR-bearing T cells expressing CD8 antigen composed of a homodimer of alpha-chains, which was not detected in other gut associated-lymphoid tissues (GALT) such as Peyer's patch, mesenteric lymph node and lamina propria tissue. The number of alpha beta T cells in these GALT increased within 1 week more quickly than that of IEL. The increase in alpha beta IEL after microbial association was not prevented by thymectomy. These results strongly suggest that the progenitors of alpha beta TcR-bearing IEL expand outside the thymus in response to microbial colonization in germ-free mice.