A rise of the intracellular serine proteinase activity (ISP) during postexponential growth of Bacillus subtilis was decreased by a temperature upshift from 35 degrees to 42 degreesC. However, the amount of both molecular forms of the major intracellular serine proteinase ISP1 determined by immunoblotting was similar at both temperatures or even slightly increased at 42 degreesC. The evolution of the ISP activity in B. megaterium showed an opposite temperature dependence, being faster during growth at 42 degreesC. The amount of immunologically detected ISP1 again did not correlate well with the enzyme activity. Moreover, most of the ISP1 molecules in cell-free extracts from B. megaterium were inactive and were activated by increasing the CaCl2 concentration up to 30 mM-unlike B. subtilis, where the enzymic activity was unaffected by Ca2+ concentration. These data suggest that the ISP1 activity in the two bacillar species during postexponential growth is regulated posttranscriptionally, but that the regulatory mechanisms differ.