The immune response of chickens to 11 isolates of LaSota strain Newcastle disease virus (NDV) and to 7 isolates of B1 strain NDV was studied in specific-pathogen-free chickens. Most of the vaccine strains were from commercial vaccine producers. Comparison of immunogenicity were made of these vaccines; five other lentogenic NDV strains, not used commercially, were also evaluated. From the criteria of hemagglutination inhibition (HI) and virus serum (VN) neutralizing titers, differences were observed with certain isolates within the LaSota and B1 vaccine groups. Whereas the majority of vaccine strains fell within a predictable range, some proved considerably less in their ability to engender expected titers. Two-to threefold differences in titers were observed with some of the strains compared. Two cloned LaSota NDV strains did not compare favorably, or with expected higher titers, with uncloned or conventional LaSota strain virus isolates. Similarly, certain B1 and LaSota strains passaged in chickens did not demonstrate a greater immunogenic potential over conventional, commercial vaccines of similar strain designation but not passaged in chickens. Whereas none of the NDV strains demonstrated evidence of neurotropism or lethality, postvaccination respiratory reactions were difficult, if not impossible, to evaluate in laboratory isolation units. The complexity of selecting ND vaccines for field use is discussed briefly.