The lymphatic absorption of four water-soluble compounds with different molecular weights (MW) was determined by measuring their cumulative recovery in lymph draining from the site of s.c. administration in sheep. The cumulative recoveries (% of dose, mean +/- SD; N = 3) were 4.0 +/- 1.5 (5-fluoro-2'-deoxyuridine, MW 246.2), 21.0 +/- 7.1 (inulin, MW 5200), 38.6 +/- 6.7 (cytochrome c, MW 12,300), and 59.5 +/- 7.7 [human recombinant interferon (rIFN) alpha-2a, MW 19,000], respectively. Our data show that in the investigated MW range, there is a linear relationship between the molecular weight and the proportion of the dose absorbed lymphatically. An increase in molecular weight results in an increased lymphatic absorption. Molecules with MW greater than 16,000 are absorbed mainly by the lymphatics which drain the application site. The knowledge gained in this investigation may help to improve the mode of administration and therapeutic efficacy of endogenous proteins whose targets are lymphoid cells (e.g., interferons, interleukins). Practical implications for the clinical use of such proteins are discussed.