In recipients of renal allotransplants the relationship of IgM, IgA and IgG serum levels to 26 rejections was investigated, as well as the relationship of IgD and 22 rejection. For IgM and IgG a significant decline was found in the whole group (p less than 0.05) and in the sub-group of rejections after a period longer than one month following transplantation (p less than 0.05 in IgM and p less than 0.02 in IgD). As compared with values before rejection, a decline of IgM occurred in 11 rejections which amounted to more than 20% and a decline of IgD in 10 rejections by more than 50%. Changes of the serum levels of IgA and IgG were not statistically significant (p greater than 0.05). In recipients of renal allotransplants serum immunoglobulin (Ig) levels of the main classes were investigated for two reasons: (1) as part of a check-up of the condition of the graft and screening of rejection and (2) to detect possible deficiencies of humoral immunity with increased liability to infections. As regards monitoring of the risk of the rejection crisis, hitherto assembled experience did not provide an unequivocal answer in which class of Ig the greatest change may be expected, whether a rise or fall, and it is not even clear whether investigations of serum Ig levels will be a positive contribution. There is only agreement on the point that IgA levels are not related to rejection; according to some authors this applies also to IgM and IgG (8, 11). Other authors (1, 15, 17) appreciate in conjunction with rejections the importance of high IgM and IgG levels, while other workers observed a significant decline of IgM (13, 16, 19). IgD levels were investigated in our previous work on the relationship of renal functions and serum Ig levels in recipients of renal allotransplants (7); we found no reports on the relationship between IgD and rejection.