We studied zero-temperature magnetic ordering in a model organic ferrimagnet-polyallyl spin-1/2 chain as a function of the exchange integral lambda describing the interaction between spins of radical centers and the main chain. Exact diagonalization and density matrix renormalization group (DMRG) methods demonstrated that for small applied fields, the difference in zero-temperature magnetizations of the sublattices reaches a maximum value at lambda = 0.70. It was shown that there is a critical value of magnetic field above which this ferrimagnetic ordering is destroyed. We also present the results of DMRG calculations that showed the gap character of excitations without decrease of the total spin for lambda greater than or equal to 0.4. (C) 2001 Elsevier Science B.V. All rights reserved.