The special cytoplasmic organelle, melanosome, formed in the Golgi region, is the site of melanin synthesis. Under electron microscopy, these melanosomes are found to assume distinct structural characteristics for various disease states such as melanoma, cellular nevi, blue nevus, nevus Ota, vitiligo, albinism, and others, providing diagnostic criteria for molecular pathology. In albino human and fish integument, melanization can be induced in vitro, using corticotropin or l-tyrosine. Electron histochemistry and radio-bioassay for tyrosinase can reveal its presence and subcellular localization in both of these tissues. The defect of albino fish appears to be a disturbed association of tyrosinase with the melanosome, while human albinism constitutes a different nature and may be the result of lack of free l-tyrosine molecules available to the melanosome, although some dermal inhibitary factors are also seen. Subcellular and cytochemical characteristics with reference to their origin are described.