Caplow (1954) and Vollmer and Mills (1966) have suggested that occupational associations, one aspect of professionalization, frequently operate so as to restrict entry to the occupation and enhance occupational incomes. Four hypotheses from the literature on occupational associations are explicitly developed and tested. No evidence is found that licensing per se affects occupational incomes. There is weak support for the argument that public representation on licensing boards tends to reduce occupational incomes. And there is rather strong support for the hypothesis that licensing is used to restrict entry and enhance occupational incomes. In 5 of 6 occupations, there is the expected negative correlation between the proportion of applicants who actually receive licenses, and occupational incomes. Finally, to the extent an occupation is professionalized, occupational income tends to be independent of across-state variations in median income.