Washed rabbit spermatozoa of freshly collected ejaculates were incubated in different fractions of seminal plasma proteins and examined for motility. Incubation media were protein fractions obtained by gel and ion-exchange chromatography. Motility was best maintained in the fractions which contained albumin, and rabbit serum albumin was more effective than bovine or human serum albumin. Concentrations of less than 4 mg rabbit serum albumin/ml resulted in markedly decreased motility.