A comparison of the proteins of ‘R’ and ‘T’ strains of mosquito iridescent virus (M1V) was made using PAGE and amino acid analysis. The banding patterns of RMIV and TM1V were similar, each showing 9 proteins. None of the proteins reacted with Schiff’s reagent. When the proteins of RMIV and TMIV were S-carboxymethylated, PAGE showed only 4 proteins for both strains compared with the 9 bands detected in control preparations. Amino acid analyses and tryptic peptide analyses indicated that the proteins of the two strains were ver