The article discusses the question of whether indeclinable forms of participles in the Old Russian Gospel manuscripts of the 11th and 12th century reflect grammatical processes of the Old Russian language of that time. The comparison of the Gospel copies of various types (tetraevangelia, short and full lectionaries) shows, firstly, that the participles used in indeclinable form are distributed according to the type of the Gospel text: most of them are used in the copies of the full lectionary, and precisely in the part that distinguishes this type from the short; and secondly, that the same contexts with indeclinable participles are reproduced in different codices of the full lectionary, while individual readings of the short lectionary and tetraevangelia are not supported by other manuscripts. This correlation allows us to hypothesize that the loss of participle declination is a linguistic feature inherited by the Old Russian manuscripts from the Bulgarian text of the Gospel. The activity of this phenomenon in full lectionary readings indicates its penetration into the text of the gospel in the first half of the 10th century — during its editing in Simeon’s epoch. The penetration of such forms in the text of the short lectionary and tetraevangelia could occur later, under the influence of the complete lectionary. Analysis of the syntactic characteristics of participle forms with the loss of agreement shows that they are acceptable in the text of the Simeon's full lectionary in all possible positions, which is a language innovation against the background of the surviving Old Slavic manuscripts.