The effect of calcium on germination of coat-modified Bacillus cereus T spores was investigated. Coat-modified spores produced either by chemical extraction (SDS-DTT-treated spores) or by mutagenesis (10LD mutant spores) were unable to germinate in response to inosine. While SDS-DTT-treated spores could germinate slowly in the presence of L-alanine, l OLD mutant spores could not germinate at all. The lost or reduced germinability of coat-modified spores was restored when exogenous Ca2+ was supplemented to the germination media. The calcium requirement of coat-modified spores for germination was fairly specific. The simultaneous presence of germinant with Ca2+ was also required for germination of coat-modified spores. The optimal recovery of germinability was obsei ved in the presence of 1.0 mm of calcium acetate. The calcium requirement itself was remarkably diminished under the condition in which L-alanine and a certain purine nucleoside analog, adenosine or inosine, coexisted. The lost or diminished germinability observed in SDS-DTT-treated spores or l OLD mutant spores may be attributed to the loss of calcium associated with the spore integuments.