From three quasar samples with a total of 1038 objects in the redshift range 1.0-2.2 we measure the variance a 2 of counts in cells of volume V(u). By a maximum-likelihood analysis applied separately on these samples we obtain estimates of sigma2(l), with l = = V(u)1/3. The analysis from a single catalog for l = 40 h-1 Mpc and from a suitable average over the three catalogs for l = 60, 80, and 100 h-1 Mpc, gives sigma2(l) = 0.46(+0.27/-0.27), 0.18(+0.14/-0.15), 0.05(+0.14/0.05), and 0.12(+0.13/-0.12), respectively, where the 70% confidence ranges account for both sampling errors and statistical fluctuations in the counts. This allows a comparison of QSO clustering on large scales with analogous data recently obtained both for optical and IRAS galaxies: QSOs seem to be more clustered than these galaxies by a biasing factor b(QSO)/b(gal) approximately 1.4-2.3.