Renal allograft recipients are at risk to acquire infectious disease complications primarily after an intensified immunosuppressive therapy. In this study, serum levels of IgG, IgA, IgM and IgG subclasses were compared between transplant patients after different anti-rejection regimen and evaluated as possible predictive markers for infectious implications. Thirty-six renal transplant recipients in the early posttransplantation period were allocated into three subgroups: group 1 consisted of 15 subjects with good primary graft function standing under a basal triple drug immunosuppression; patients of group 2 (n = 9) had a very early acute rejection episode which was treated by high steroid doses and patients of group 3 (n = 12) required antithymocyte globulin (ATG) therapy for acute rejection. Immune parameters were studied 5 weeks after transplantation, when basal triple drug immunosuppression was continued in all patients. Twenty-three age-matched patients under chronic hemodialysis were recruited as a control group. Total IgG serum levels were reduced in transplant patients compared to dialysis subjects, whereas IgA and IgM levels were not altered. IgG subclass analysis revealed, that IgG1 levels were similar in stable transplant compared to chronic hemodialysis patients, but were significantly reduced in rejection patients after steroid pulses or ATG therapy. IgG3 levels were significantly reduced in all transplant recipients compared to dialysis patients without differences between the transplant subgroups. IgG2 and IgG4 subclass levels were similar in all studied subjects. IgG subclass abnormalities were paralleled by a reduction of lymphocyte subsets in rejection patients of groups 2 and 3, which may be the basis for an altered Ig class switching. Seven out of 36 studied transplant patients had severe infections in the early posttransplantation period with four cases of viral infections. Patients with infections had significantly lower IgG1 levels and CD4 positive lymphocyte numbers but similar total IgG levels compared to patients without infections. So IgG1 levels may be of value to predict the occurrence of infectious complications in renal allograft recipients in the early posttransplantation period.