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Summary Twelve vaccine batches prepared from avirulent vaccine strains of Newcastle disease virus produced by seven manufacturers were identified by analysis of the matrix (M ) protein gene with restriction enzymes Mbo I and Hin fI. The analyses have revealed the presence of the strain indicated by the manufacturers (namely B-1, LaSota or Ulster 2C), except in one case when the vaccine contained strain V4 Queensland instead of VGGA as indicated. In addition, several batches of both monovalent and combined vaccines containing strain LaSota of the same company consistently disclosed contamination with strain B-1. The mixed nature of the preparations was verified not only by the dual patterns of restriction fragments but also by separating the two components and identifying them individually. Restriction analysis of the M gene, by allowing positive identification of each of the lentogenic vaccine strains, should provide an improvement in controlling vaccine batches by revealing homologous contaminants or exchange of the vaccine strain. [ABSTRACT FROM AUTHOR]