1. Homocysteine which is formed during the metabolism of methionine is readily oxidized and is measured by the amino acid analyser as cysteine—homocysteine mixed disulphide and homocystine. We measured plasma amino acid concentrations after an overnight fast in 27 stable long-term renal transplant recipients and 25 age-and sex-matched normal subjects with particular emphasis on sulphur-containing amino acids. 2. Plasma cysteine—homocysteine mixed disulphide was increased in the patients (mean 6.0 ± sd 3.2 μmol/l; normal 3.1 ± 0.9 μmol/l, P < 0.001) and homocystine was detectable in low concentration (< 1.0 μmol/l) in 24; the elevation in cysteine—homocysteine was related to serum creatinine ( r = 0.60, P < 0.002). Cystine was also increased (91.6 ± 29.3 μmol/l; normal subjects 64.0 ± 16.7 μmol/l, P < 0.001), but methionine concentrations were normal. 3. When pyridoxine, folic acid and vitamin B 12 , cofactors for homocysteine metabolism, were administered sequentially to 11 arbitrarily selected transplant recipients cysteine—homocysteine decreased from 7.3 ± 2.1 to 4.3 ± 0.8 μmol/l ( P < 0.001) and homocystine became undetectable. the response coincided with the giving of folic acid and occurred without alteration in serum creatinine and with normal serum folate and vitamin B 12 concentrations. 4. in eight patients in whom pretreatment erythrocyte folate was measured, folic acid therapy reduced cysteine—homocysteine from 9.0 ± 3.1 to 5.4 ± 1.6 μmol/l over a 4 week period ( P < 0.001), the largest response being in the one patient with subnormal erythrocyte folate; values were in the low-normal or normal range in the other seven. 5. We conclude that plasma homocysteine is increased in renal transplant recipients when serum creatinine is only moderately elevated and that the homocysteine concentrations are decreased by treatment with folic acid, suggesting that both reduced homocysteine excretion and relative shortages of folic acid are responsible.