Using flow cytometry, 32 kidney graft recipients were monitored retrospectively for at least 1 month to study the dynamics of serum IgG, IgA and IgM antibodies against donor T and B lymphocytes before and after transplantation. Donor spleen lymphocytes were used as targets. In the B cell cross-match, the surface immunoglobulins were blocked with an anti-human immunoglobulin antibody. A high frequency of donor-reactive antibodies was detected early after transplantation, especially when the sera were tested against B lymphocytes. Surprisingly, donor-reactive antibodies of the IgA isotype made up a substantial proportion of all antibodies detected. Within the first month after transplantation, six out of 32 patients (19%) developed IgG antibodies against donor T lymphocytes and nine out of 35 patients (28%) formed IgG antibodies against B lymphocytes. A similar situation was found for IgA antibodies: 22% of the recipients were positive for IgA antibodies against donor T lymphocytes and 34% against B lymphocytes after transplantation. Lower antibody frequencies were found for IgM antibodies (16% and 19%, respectively). From our data we conclude that for at least some of the parameters studied their individual dynamics reflect the complex immunological mechanisms occurring early after transplantation.