This paper combines new CCD polarimetric data with previous information about protostellar objects in a search for correlations involving the interstellar magnetic field (ISMF). Specifically, we carried out an optical polarimetric study of a sample of 28 fields of 10'x10' located in the neighborhood of protostellar jets and randomly spread over the Galaxy. The polarimetry of a large number of field stars is used to estimate both the average and dispersion of the ISMF direction in each region. The results of the applied statistical tests are as follows. Concerning the alignment between the jet direction and the ISMF, the whole sample does not show alignment. There is, however, a statistically significant alignment for objects of Classes 0 and I. Regarding the ISMF dispersion, our sample presents values slightly larger for regions containing T Tauri objects than for those harboring younger protostars. Moreover, the ISMF dispersion in regions containing high-mass objects tends to be larger than in those including only low-mass protostars. In our sample, the mean interstellar polarization as a function of the average interstellar extinction in a region reaches a maximum value around 3% for A(V)=5, after which it decreases. Our data also show a clear correlation of the mean value of the interstellar polarization with the dispersion of the ISMF: the larger the dispersion, the smaller the polarization. Based on a comparison of our and previous results, we suggest that the dispersion in regions forming stars is larger than in quiescent regions.