SUMMARYTwenty‐six combinations of C57BL/10 congenic mice involving incompatibilities in the I region and the H‐2K or H‐2D regions of the major histocompatibility complex were studied in a cell‐mediated lympholysis assay (CML). All combinations were positive; however, six were unidirectional. Three of these were consistent with the mapping of an effector cell stimulating locus (ECS) between the IB and IC sub‐regions. Possible reasons are given for the negativity of CML in I/H‐2K or H‐2D region incompatible combinations. A statistical method used to analyse the data