De los Reyes took home with him works by socialists such as Karl Marx, Proudhon, Bakunin, and Errico Malatesta. Malatesta's "Propaganda socialista fra contadini" was particularly familiar to union organizers. The UOD was the first labor federation in the Philippines, soon being joined by neighborhood associations from Cavite, Quiapo, Santa Cruz and Sampaloc; company guilds from the San Miguel Brewery and L.R. Yangco Shipping Company; and trade associations of printers, "tabaqueros", tailors, sculptors, seamen, and cooks. At its peak in 1903, the UOD's membership was estimated at around twenty thousand.