The changes in the concentrations of total, nonprotein and protein SH-groups were spectrophotometrically determined in the small intestine and bone marrow of the rats 10, 20, 30 and 60 min after the administration of protective doses of cystamine (50 mg/kg), mexamine (10 mg/kg) or of a mixture of cystamine with mexamine (20 + 10 mg/kg). Cystamine caused an increase in the concentration of mainly nonprotein thiols, whereas the mexamine administration increased the protein SH-groups (mainly between the 20th and 30th min after treatment). The radioprotective mixture led to the most pronounced increase at the 10-min interval. This increase at the 10- and 20-min intervals was more pronounced (or identical) than after cystamine or mexamine administered as a single agent. If the SH-concentration should correspond to the degree of radioprotection, then the phenomenon observed would indicate the usefulness of the mixture administered.