Abstract A histochemical fluorescence method for the demonstration of biogenic monoamines was applied to the smear preparation of peripheral blood platelets and bone marrow megakaryocytes of rabbits and humans. The fluorescence obtained was identified as 5-HT by histochemical and pharmacologic criteria. With this technic, the following results were obtained: (1) A large amount of 5-HT was present in platelets and in mature platelet-forming megakaryocytes. (2) Only a small amount of 5-HT was demonstrable in the intermediate maturation forms of megakaryocytes with lack of platelet budding. The possibility that the 5-HT detected was derived from (a) transport of 5-HT formed elsewhere, and/or (b) 5-HT formation from 5-HTP in the megakaryocytes themselves during their maturation was discussed.