Nine NDV strains belonging to lentogenic, mesogenic and velogenic groups were studied. Virus adsorption to chicken red blood cell (RBC) surface was performed at 4 degrees C, and after a temperature shift from 4 degrees to 37 degrees C elution of pre-adsorbed virus and accumulation of free N-acetyl-neuraminic acid (NANA) split from RBC receptors as a result of neuraminidase (Nase) activity was detected. In the case of high multiplicity of adsorption the elution was very fast (complete elution within 5 minutes) for all the strains irrespective of their virulence. Although physical saturation of RBC surface with the adsorbed virus was not achieved, a certain minimal (strain-specific) amount of the pre-adsorbed virus which splits a maximally possible (for a given strain) quantity of the NANA was found (a state of "enzymatic saturation"). Below a certain low multiplicity of adsorption elution was delayed for about 20-30 minutes while the accumulation of the split NANA began immediately after the temperature shift. This phenomenon was interpreted as a result of "crawling" of the adsorbed virions upon the RBC surface followed by "browsing" of RBC receptors and liberation of NANA. Thus, the Nase activity of the attached virus ("in situ Nase activity") is a factor providing both elution and "crawling" of the virus (depending on the multiplicity of adsorption). The in situ Nase activity of all the strains used was determined quantitatively by (1) parameters of enzymatic kinetics (Vmax, Km and Km/Vmax) and (2) parameters of enzymatic efficiency related to a certain quantity of the adsorbed virus, namely, per amount of: a) "crawling" virus, b) that providing "enzymatic saturation", and c) that equal to Km. Computation of these parameters revealed inverse correlation between the in situ Nase activity and the strain virulence. Thus, these indications can be in vitro markers of the in vivo virulence.