Bovine follicular fluid was compared with blood serum or blood plasma with respect to the concentration of protein and free amino nitrogen, the number of antigens and the number and mobilities of electrophorettc components. The protein concentration of follicular fluid (7.08 g/100 ml) was less than that of blood serum (9.10 g/100 ml), but no significant difference existed between the values of free amino nitrogen in the 2 fluids (4.31 and 3.97 mg/100 ml, respectively). Rabbit antisera to follicular fluid, blood serum or blood plasma gave several precipitin lines (at least 7, 7 and 8 respectively) when reacted with their homologous antigens in agar-gel-diffusion studies. After absorption with blood serum, there was one antigen (presumably fibrinogen) in blood plasma and follicular fluid that was not found in blood serum. These results were substantiated by immuno-electrophoresis. Moving boundary electrophoresis in 5 different buffers revealed at least 8 components in follicular fluid and blood serum and at least 9 in blood plasma. Minor differences were observed in the electrophoretic mobilities of some components found in both follicular fluid and blood serum. The results showed that the major macromolecular components of follicular fluid and of blood were similar, but that some minor macromolecular ones may differ.