Tyrosinase activity was compared in the skin and hair bulbs of young black and agouti mice between 4 and 12 days old. Differences in activity were found to be maximal in both the hair and skin at the time of yellow pigment synthesis in agouti mice. Histological examination suggested that the number of dopa‐positive melanocytes is similar in the hair bulbs of agouti and black mice. The level of SH‐compounds in the hair bulb was examined and found to be elevated in agouti tissue at the time of phaeomelanin formation. It was shown that sulphydryl compounds such as cysteine and glutathione have an inhibitory effect on tyrosinase, and it is possible that the elevated levels of SH‐compounds are responsible for a reduction in tyrosinase activity in agouti mice. In agouti hair bulbs, this effect can be reversed in vitro by addition of c
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