Discs containing the pteridine compound O129 at various concentrations may be useful in differentiating the following closely related species belonging to the genus Bacillus, viz. B. subtilis, B. licheniformis and B. pumilus; B. polymyxa and B. macerans; together with B. cereus and its subspecies B. cereus var. mycoides. At concentrations of 10 micrograms O129, the ATCC type strain of B. subtilis was resistant whereas B. licheniformis and B. pumilus were sensitive. However, B. subtilis was sensitive to O129 at 150 micrograms. Similar reactions differentiated the ATCC type strains of B. polymyxa and B. macerans. The ATCC type strain of B. cereus was resistant to O129 at both 150 micrograms and 10 micrograms, but its subspecies B. cereus var. mycoides (ATCC 28) was partially sensitive at 150 micrograms concentration. When clinical isolates of B. cereus var. mycoides were subsequently tested, this partial sensitivity was found to be a variable characteristic.