Lipoproteins were separated by centrifugation and column chromatography and included in citrate-based semen diluents. One lipoprotein fraction was particularly effective in preventing injury to bovine spermatozoa during dilution and freezing and thawing when assessed using motility and release of hyaluronidase as criteria of cell damage. Radioactive labelling of egg-yolk lipids demonstrated that egg-yolk components remained associated with spermatozoa even after extensive washing to remove diluent. The patterns of labelling of lipids extracte from the washed spermatozoa did not indicate that particular lipids became associated with spermatozoa. Increases in the specific radioactivity of lipid extracts from washed spermatozoa lent support to the contention that lipoproteins become firmly bound to the cells.