R. L. KAURA and S. GANAPATHY IYER                         205

strain of the same virus on 20th April 1936 (Table IV). Pigeon No. 344 re-
sisted both the strains and was, therefore, naturally immune to this infection,
whereas, the other four pigeons reacted typically to the Indian strain of the
virus which suggested that there was some immunological difference between
the two strains (Plate X, Figs. 1 and 2).

Four more pigeons, after recovery from experimental infection with the
English strain of pigeon-pox virus were submitted to immunity test against
the Indian strain of the same virus on 12th May 1936 (Table IV). Of these,
three pigeons showed no reaction but the fourth one, No. 35, reacted mildly.

Since the results of the above two experiments were not similar, a fresh
lot of four pigeons which were previously infected with the English strain
of pigeon-pox virus were tested with the Indian strain of the same virus on
23rd May 1936 (Table IV). Pigeon No. 12 died three days after the cross
inoculations due to other causes and the other three proved refractory.

In continuation of this work seventeen more pigeons, actively immune to
the English strain of pigeon-pox, were inoculated with the Indian strain
of the same virus on 13th July 1936. Of these, fifteen pigeons reacted and the
remaining two showed no reaction.

From the results of the above experiments it appears that cross immunity
does not always exist between the English and the Indian strains of pigeon-
pox virus and there seems to exist some difference in the antigenic complex
of the two strains.

                                        TABLE IV

Cross immunity test on pigeons, immune to the English strain of pigeon-pox
                virus against the Indian strain of the same virus