Immunization of Macaca irus monkeys with purified soluble HL-A substances provokes both antispecies and anti-HL-A cytotoxins. By absorption with defibrinated human blood, HL-A reagents of high titre can be prepared in good yields based on initial cytotoxicity. The absorbed sera have a very high correlation with HL-A specificity. When a monkey was immunized with an HL-A substance for which its peripheral lymphocytes typed positively, neither HL-A nor species cytotoxins were produced. A hypothesis is discussed to explain this finding in which it is proposed that a species determinant on the HL-A molecule represents a weaker immunological stimulus to another primate species than the HL-A determinant.