A panel of over 100 individuals was tested against several 4a and 4b antisera. A 4a/4b type was assigned to each individual on the basis of overall reactions with each group of sera. It was felt that because 4a and 4b antisera rarely give identical reaction patterns the use of several sera in each group gave a better definition of the specificities.The relationship between each second locus HL‐A specificity and 4a or 4b was assessed. Most of the “narrower” antigens showed complete inclusion within one group but there were some anomalies. Associations of these specificities were analysed further by absorption experiments.4a and 4b antisera were able to define differences in some second locus specificities which were not picked up by the narrower antisera. It seems likely that 4a and 4b represent a separate public site on the HL‐A gene product which is multifactorial. For a full description of the antigens all factors should be stated as there is not an invariant association between com