The proaggregatory potencies of synthetic analogs of platelet activating factor (PAF-acether: 1-0-alkyl 2-0-acetyl sn-glycero-3-phosphorylcholine) were tested in rabbit and human platelet rich plasma (PRP). For activation of human compared to rabbit platelets about 30 to 100 times higher concentrations of the compounds were needed, but the relative potencies of the analogs tested were quite similar in both species. The absolute stereospecificity required for platelet activation could be confirmed and extended also to the pair of enantiomers with a cis-9 double bond in the 0-alkyl chain and to the enantiomers with the 0-alkyl chain at C-2 of glycerol. The propionyl ester at C-2 of glycerol proved to be as potent as the acetyl ester but replacement by the formyl as well as by the butyryl ester reduced the activity to less than three per cent. Variations at the 3-phosphorylcholine substituent all abolished the activity. It appears that the correct stereochemistry and a phosphorylcholine group in a strictly defined position towards the asymmetric C-2 of the glycerol backbone are absolute requirements for activity whereas variations in the alkyl and the short chain acyl groups have not as dramatic consequences.