Spores of Bacillus subtilis show similar plating efficiency on media with or without 1.5 M NaCl. In contrast, vegetative cells are osmosensitive unless the stationary phase has been reached. In the present work, loss of heat and osmotic resistance during germination was studied. Their kinetics and sensitivity to protein synthesis inhibition were different: heat resistance was lost first and even in the presence of chloramphenicol, whereas loss of osmotolerance occurred later and was inhibited in the presence of this antibiotic. The influence of spore-associated small acid-soluble proteins (SASPs) on spore osmotolerance was investigated using ssp mutants: all produced spores which germinated poorly and were sensitive to osmotic strength. SASP-E deficiency was particularly significant. Spore osmotolerance was largely restored in complementation assays performed with cloned ssp genes. It is possible that germination-associated degradation of SASP proteins provides osmotically significant levels of amino acids (especially glutamate).