Soluble HL-A antigens can be extracted with high yields from whole human platelets or platelet membrane preparations by the non-ionic detergent NP-40. The active molecules have high molecular weights (⩾ 200 000) and the HL-A specificities present in the preparation were not separated by electrofocusing. An increase of the detergent/protein ratio gives rise to active molecules with lower molecular weight (15 000–30 000). In this case the two specificities studied (HL-A2 and HL-A7) could be separated by electrofocusing.