We describe here several improvements in the method we originally developed to prepare mitotic chromosomes from peripheral blood of laboratory mice. In addition, we have tried several methods to improve metaphase yield from lymphocytes of the inbred strain DBA/2J, which respond poorly to phytohemagglutinin. The yield of mitoses from DBA/2J cells cannot be improved by enhancing T-cell response using interleukin-2 or by using a different T-cell mitogen, concanavalin A. Metaphase yield from peripheral blood cells of DBA/2J mice can be improved significantly by adding lipopolysaccharide to cultures, probably stimulating B-cell as well as T-cell proliferation.