The lymphocytic infiltration found in multiple cultured BALB/c thyroids placed in the same C57BL/6 recipient was found to be highly correlated and the high variability between animals was not influenced by the number of lobes. It was concluded that the variable infiltration was largely due to host factors. Because a similar correlation was found between BALB/c and C3H grafts, the response to a minor antigen common to these two strains was suggested as a cause of the infiltration. When the response of C57BL/6 mice to cultured B6.C (H-2d) and BALB.B (H-2b) grafts was compared, a synergism between major and minor antigens was suggested. However, when the time of culture was increased from 48 to 60 h and the response of BALB/c and C57BL/6 mice were compared with identically cultured B6.C (H-2d) grafts, a striking difference between major and minor antigens was observed. None of 10 such grafts in C57BL/6 recipients (major antigens only) showed any infiltration, whereas 8 out of the 9 grafts in BALB/c recipients (minor antigens only) were infiltrated.