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Experiments were conducted to test the effect of egg yolk fractions, bovine serum albumin (BSA), and ovalbumin on washed immotile sperm. Egg yolk contained several fractions that initiated 'l1otility in washed immotile sperm, and one of these was water-soluble and heat-stable. These motilityinitiating fractions were found in the retentate of dialysed egg yolk. Egg yolk also contained compound(s) located in the diffusate of dialysed seminal plasma that sustained motility in activated samples. The nature of the sustaining compound(s) is not known. Washed immotile sperm could be readily reactivated with egg yolk after up to 4 days' storage, but not after storage for 8 and 16 days. Failure at Day 8 occurred when 54% of the sperm were still alive. Many of these sperm were active after 24 h incubation when EDTA was present in the storage diluent, but not when EDT A was absent. It was concluded that delay in activation resulted from the time taken for critical concentrations of sustaining compounds to diffuse into the cell. The reason for the failure to reactivate immotile sperm stored without EDT A is not known. BSA was more effective than ovalbumin in initiating motility in washed sperm. However, the concentration of BSA required to initiate motility varied between samples, suggesting that sperm from different bulls may vary in their requirement for initiating compound(s). It is concluded that the property of motility initiation resides in several large molecular weight substances, though the efficiency of initiation of these substances may vary greatly. Efficiency of initiation of motility was not related to the effect of the initiating compound on the permeability of the cell membrane to sustaining compound(s).

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