Before the uptake of water that precedes spawning, eggs of cod (Gadus morhua L.) contained 30% dry matter, of which 80% was protein. Some 75% of this protein was soluble in 0.5m-sodium chloride. The major components in the extract were two similar lipoproteins, of molecular weight about 400000, containing 21% lipid, some two-thirds of which was phospholipid, and about 0.5% protein phosphorus. 2. These lipoproteins were identified by immunochemical methods in the serum of female cod with developing ovaries, but not in the serum of male or of immature female fish. 3. The concentrations of egg proteins in the serum of female cod were determined by a serial-dilution double-diffusion immunological method, and were shown to increase with development of the ovaries, reaching a value of about 32mg/ml when the weight of the ovaries was 10% of the weight of the fish. 4. Immature male and female cod were injected intramuscularly with a solution of oestradiol-17beta 3-benzoate in oil and the concentration of egg proteins in their serum was measured by the immunodiffusion method. The serum contained no detectable egg proteins before injection of the fish, but 30mug of oestradiol benzoate/kg gave rise to detectable amounts of egg proteins in 10 days, and with 300mug or 1mg of oestradiol benzoate/kg the concentration of egg proteins rose to 32mg/ml. The values for male and female cod were similar and represented about one-half of the total serum protein. 5. With a dose of 1mg of oestradiol benzoate/kg, egg proteins were first detected in the serum 2 days after injection and the concentration increased up to 10 days. 6. Serum samples taken before and 10 days after an injection of 1mg of oestradiol benzoate/kg were fractionated by gel-filtration on Sephadex G-200. The difference curves obtained from fractionation curves after and before injection confirmed the values of the concentrations of egg proteins obtained from the immunodiffusion test and showed that the concentrations of the normal serum components fell by 20-50% of the initial value, the high-molecular-weight globulins showing the most marked fall. 7. Egg proteins were detected in the liver and testes of the injected fish, but not in the ovaries.
