Immunoglobulins (Ig) and complement, which are components of humoral immunity, are supposed to play a role in renal transplant rejection. The present study was undertaken to study the level of complement C3, C4 and IgG, A and M in patients with chronic renal failure (CRF) and in those with renal transplant rejection (Tx Rej) as well as stable transplant recipients (Tx Stb) and normal healthy controls (NHC) in order to assess their role in transplant rejection and to correlate them with histopathological findings. The mean level of C3 and C4 in the CRF, Tx Rej and Tx Stb groups was not significantly different from the NHC group (P > 0.05). The mean level of C3 in the Tx Rej group was not different from that in the Tx Stb group. However, the C4 level was significantly reduced in the Tx Rej group when compared with the Tx Stb group (P < 0.05). There was no histopathological correlation between C3 levels and acute cellular rejection (ACR) or chronic allograft nephropathy (CAN); however, C4 levels were reduced in about 50% of the cases with CAN. The mean serum IgG level was significantly reduced in patients with CRF and transplant recipients as compared with NHC. The serum IgA level was also significantly reduced in Tx Rej cases. Correlation of serum IgA with histopathology in cases with rejection showed that in ACR, a lower mean level of IgA was seen as compared with that seen in cases with CAN. The serum IgM level was significantly higher in the Tx Rej group as compared with the Tx Stb group. There was no significant correlation between serum IgM levels and renal histopathology in patients with ACR and CAN. The C3 level showed a significant positive correlation with IgG (r = +0.50, P < 0.05) in the Tx Stb group. This study shows that cell-mediated immunity is the main cause of rejection in both ACR and CAN while humoral immunity is also involved along with cellular immunity in some cases with CAN.