The date of onset of 360 acute renal transplant rejection episodes from 1969 to 1973 have been compared with the prevalence of various common viral infections and infections due to Mycoplasma pnuemoniae. A positive correlation was found for influenza B infections (r=0.43, p less than 0.01) up to 5 months before transplantation and for adenovirus infections (r=0.32, p less than 0.05) at 1 month before kidney grafting.