Cytomegalovirus (CMV) specific immunoglobulin G (IgG) and immunoglobulin M (IgM) antibody responses were measured before and after renal transplantation in 20 patients with primary CMV infection and in 16 patients with recurrent CMV infection. In primary CMV infection IgG antibody titres to late antigen (IgG-LA) measured by indirect fluorescence (IFA) were approximately seven times higher than those obtained by the complement fixation test (CFT). In contrast, in recurrent CMV infection this difference was found to be about twofold. Virus-specific IgM antibody to late antigen (IgM-LA) was detected in 100 percent of patients with primary CMV infection and in only 50 percent of patients with recurrent CMV infection. The IgM-LA titres were highest in primary CMV infection and reached peak levels at approximately 10 weeks post transplantation, whereas in recurrent CMV infection the IgM-LA titres were lower and reached peak levels at three months post transplantation. Moreover, IgM-LA was found to persist in patients from both groups at nine months post transplantation. IgM antibody to early antigen (IgM-EA) was not detected in any patient in this study. However, significant fourfold titre rises in IgG antibody to EA (IgG-EA) were detected in 100 percent of patients with recurrent CMV infection and in 50 percent of patients with primary CMV infection. These results clearly show the difference in antibody responses to the various antigens of CMV in patients with primary and recurrent CMV infection.