Abstract β-Mercaptolactate cysteine disulfide (βMLCD) was detected in two mentally normal sisters, 11 and 13 years old. After isolation by ion-exchange chromatography, high-voltage electrophoresis, and adsorption chromatography on Porapak Q, βMLCD was identified by mass spectrometry of its following methyl ester derivatives: O, N-di-trifluoracetyl, O-trifluracetyl- N-dinitrophenyl, O, N-diacetyl, and O-acetyl- N-dinitrophenyl, with acetyl groups 50% perdeuterated, as well as by gas chromatography-mass spectrometry combination of the desulfuration products alanine (as N-trifluotacetyl-methy; ester) and lactic acid (as methyl ester). The isolated βMLCD contained no sulfoxide nor sulfone and exhibited an UV spectrum closely related to cystine. In urine the concentration range of βMLCD was 345–751 μmole/1 ( n=5) and 54–133 mg/g creatinine; in plasma a trace of βMLCD could be detected only in one case. Oral administration of L-cysteine or L-methionine elevated the concentration of βMLCD in urine and in plasma. A further disulfide accompanying βMLCD in urine was isolated and identified as thioglycolate cysteine disulfide.