Under defined conditions, in the presence of 10 mg/ml of bovine serum albumin, cauda epididymal rat spermatozoa displayed vigorous motility, and a high proportion (81%) of eggs were fertilized. In contrast, no fertilization was observed after omission of albumin, or replacement of the protein by 10 mg/ml of cytochrome c, beta-globulin, gamma-globulin, hemoglobin, lysozyme, and polyvinylpyrrolidone, and 5 mg/ml of ribonuclease. However, high motility occurred in suspensions containing 3 x 10(6) spermatozoa/0.1 ml of medium with cytochrome c, beta-globulin, or gamma-globulin. In medium with 1 mg/ml of ovalbumin, 7% (2/29) eggs were fertilized. Use of defatted albumin resulted in a higher rate of fertilization than unmodified albumin (87 vs 70%), and this difference approached statistical significance. No fertilization was obtained in the presence of albumin presaturated with cholesterol. These results suggest that: (a) rat sperm cells failed to capacitate in the absence of albumin; (b) the protein exerted more than a nonspecific macromolecular effect; and (c) lipids associated with albumin may modify its ability to promote sperm capacitation.