as steroids, and the protein renders the medium undefined. To overcome these problems, we have investigated the effect of eliminating the oil layer and substituting polyvinylpyrrolidone (PVP) for the protein. Oocytes were collected from 12to 15-week-old Swiss mice as described by Biggers et al. (1967) and groups often were placed in each well of a Microtest Plate (Falcon Plastics, Cat. No. 3034). Previously, the wells had each received 20 μ of medium, consisting of a modified Krebs-Ringer bicarbonate (NaCl, 119-39; KC1, 4-78; CaCl2.2H20, 1-71; KH2P04, 1-19; NaHC03, 25-07 dim) described by Brinster (1965), supplemented with sodium pyruvate (0-25 μ) and crystalline bovine serum albumin (1 mg/ml). Potassium penicillin G (50 U/ml) and streptomycin sulphate (50 /?g/ml) were added to safeguard sterility. When this culture system was incubated under 5% C02 in an air atmosphere, the oocytes formed polar bodies but with significantly lower efficiency than in drops under oil. Since the oil layer might provide a lower oxygen concentration by restricting gaseous exchange between atmosphere and medium, the effect ofvarying the oxygen concentration of the atmosphere on the proportion of oocytes that developed first polar bodies was studied. Microtest Plate cultures were placed in small desiccators, which were evacu¬ ated to 60 mm Hg and refilled five times with atmospheres containing the appropriate partial pressure of oxygen. The balance of the atmosphere was nitrogen, except for C02 which was kept constant at 5 %. After incubation for 18 to 20 hr at 37° C, the proportion of oocytes which had formed polar bodies was recorded. Text-figure 1 shows that when no oxygen was added to the atmosphere, relatively few oocytes produced polar bodies. Maximum matura¬ tion, not significantly different from that observed under oil, with minimum variation occurred with 5 to 10% oxygen. Above this concentration, maturation