Spore proteins of the microsporidian Nosema bombycis, from the silkworm Bombyx mori, were analysed by SDS–polyacrylamide gel electrophoresis. The protein profile of partially solubilized spores showed three major peptide bands of molecular weight 68, 94 and 100 kDa. On complete solubilization, it showed peptide bands ranging from 17 to 68 kDa. Attempts to purify the 17 kDa infection-specific protein showed aggregation of this protein to higher molecular size proteins. Partial peptide analysis of the different peptides exhibited similar patterns suggesting the probablity of processing during the infective cycle. Reconstitution assay showed the reversible nature of this processing. N-terminal sequencing showed homology to heat shock proteins. The low molecular weight 17 kDa protein also showed very high protease activity.