The GR9 tumour was induced with methylcholanthrene in a BALB/c mouse, adapted to tissue culture and cloned without any passage in vivo GR9 clones were typed for H-2 with three monoclonal antibodies that define H-2Kd + Dd, Kd and Dd antigens. A great heterogeneity of H-2d expression was found from clones which were Kd and Dd positive to clones Kd and Dd negative. These results were confirmed for A7 and B9 clones using immunoprecipitation with anti-H-2D.4 and anti-H-2K.31 alloantisera and SDS-PAGE analysis. In addition, the number of chromosomes per cell was heterogeneous amongst the clones, ranging from 38 +/- 2 to pseudotetraploid clones which have 75 +/- 2 chromosomes. GR9 clones were injected into syngeneic BALB/c mice to measure local tumour growth. We found that the growth correlated with the amount of H-2 antigen expressed, i.e. clones with low H-2d expression were highly malignant while clones with normal H-2d expression were highly immunogenic. Finally we found that BALB/c mice immunized against A7 (Kd, Dd-positive) protected against A7, as expected, but surprisingly also against B9 (Kd, Dd-negative).