Regular 4-weekly follow-up controls of serum lactate dehydrogenase activity in 23 renal transplant recipients revealed a constant rise in serum LDH activity during the early postoperative months. During the first post-transplant month serum LDH activity increased from 150.0 +/- 48.2 mE/ml to 195.5 +/- 84.8 mE/ml, serum enzyme activity being highest (329.1 +/- 143.2 mE/ml) 6 months after surgery. Since serum creatinine levels remained relatively constant, it seems unlikely that renal rejection played a major pathogenic role in the production of increased LDH activity. Since the pattern of lactate dehydrogenase isoenzymes was ithin normal limits, the pathogenesis of increased LDH serum activity following renal transplantation is not yet clear. Possible causes such as liver damage due to hepatitis B, macrocytosis induced by immunosuppressive therapy and myopathy to steroids are discussed.