1. Antisera, prepared against extracts of cells and spores of Bacillus subtilis, were used in immunoelectrophoretic studies of the changes occurring in cell extracts during the course of spore formation. 2. At least 15 antigens could be detected in vegetative-cell extracts by the antiserum prepared against cell extracts and at least seven could be demonstrated in spore extracts by the homologous antiserum. 3. Cross-absorption studies showed that two of these antigens were probably completely specific for vegetative-cell extracts and that one was probably completely specific for spore extracts. The remainder were probably present in very small quantities in the heterologous extract. 4. In extracts of cells sporulating in an ‘exhaustion medium’ those antigens characteristic of the spore began to appear about 1hr. after the end of exponential growth. 5. In cells sporulating in a resuspension medium, spore antigens were detected at 4hr., and by 7hr. a decrease in vegetative-cell antigens was observed. 6. In an asporogenous mutant blocked early in sporulation there was neither an increase in spore antigens nor a decrease in vegetative-cell antigens. 7. In an asporogenous mutant blocked later in sporulation, there was an increase in spore antigens similar to that which occurred in the sporogenous strain.

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