Responses of approximately 200 persons were measured by mixed leukocyte cultures against 41 lymphoblastoid cell lines which were thought to be homozygous for HLA—D (i.e. LCL‐HTCs). These responses were standardized by computer analysis and the resultant Interaction Indices for each LCL‐HTC were compared with those of every other LCL‐HTC in a correlation matrix of 861 independent analyses. Only those analyses involving LCL‐HTCs of defined HLA—D types are reported here. In general, these studies confirmed many allelic relationships of reputedly similar LCL‐HTCs; however, several unexpected groups of correlations suggested that groups Dw1 and 3 overlap to form Groups “1a‐3,”“1b‐3” and “true 3.” The “1b‐3” group also included Dw8. Segregation of the “1b‐3” allelic group was also observed in a family study. These preliminary analyses suggest a working model for further investigation of HLA‐D allelic relationships (Fig. 3). On the other hand, they may also be interpreted to suggest that “typing” with LCL‐HTCs may be controlled by a separate HLA locus whic