Measurement of plasma concentrations after the oral administration of S-carboxymethylcysteine in two different dosage forms, as a hard gelatin capsule and as a syrup, shows its relative bioavailability from the two formulations to be similar. With the exception of the time required to reach peak concentration, which shows a slight variation attributable to the time taken for the drug to be released from the capsule and to dissolve, parameters such as peak concentration, biological half-life and area under the serum-concentration curve whether from t = 0 to t = 8 h or from t = 0 to t = infinity, are not statistically different. The areas under the plasma-concentration curves between t = 0 and t = 8 h and between t = 0 and t = infinity shows that for a confidence limit of 95 per cent, the mean for the capsular form does not differ by more than 3.3-10.3 per cent from the mean for the syrup.