We determine a subring of the Chow ring and the cohomology of BP GLn, the classifying space of the projective linear group of degree n over complex numbers, and explain a way in which this computation might play a role in the period-index problem. In addition, we show that the Chow ring of BP GLn is not generated by the Chern classes of linear representations of P GLn. Contents 1. Introduction 1 2. Preliminaries on motivic homotopy theory 5 3. The classes ρ p,k and y p,k 15 4. On the cohomology of the extraspecial p-groups p 1+2r + 17 5. A non-toral p-elementary subgroup of P U p r 20 6. A polynomial relation 26 7. The period-index problem 31 8. On the Chern subring of CH * (BP GL n ) (p) 33 Appendix A. A partial Jacobian criterion over perfect fields of positive characteristics 33 References 34Notations.