A survey of sweet itch in horses in Israel based on a questionnaire to owners reported that 158 of 723 horses (21.8 per cent) had sweet itch lesions. The results indicated that the likelihood of a horse acquiring sweet itch decreased with increasing altitude but no definite association with rainfall zones was evident. Variation in the density of the horse population, however, obscured these observations. In the population surveyed, stallions were more sensitive than mares and pale horses appeared to be less sensitive than dark ones, but the sample size of this latter group was much smaller. Intradermal injection of extracts of Culicoides imicola and Stomoxys calcitrans gave immediate reactions in sensitive horses but delayed reactions were observed only with extracts of C imicola. Sensitivity to extracts of C circumscriptus was also evident in allergic horses. Antibodies to extracts of Culicoides species and Stomoxys species were demonstrable in the serum of normal and allergic horses by the ELISA technique.

Uridine-specific IgG and IgM antibodies, obtained by immunization of rabbits with a uridylated synthetic polypeptide, were isolated and purified by the use of a uridine-acetyl cellulose conjugate as immunosorbent. The IgM antibodies were separated from the IgG antibodies by gel filtration on Sephadex G-200. Both antibody preparations were pure by ultracentrifugation and immunoelectrophoresis criteria. The precipitating activities of IgM and IgG antibodies were similar on a weight basis, although the equivalence zone in the case of IgM antibody is broader. I n spite of its inability to precipitate the homologous antigen, the reduced alkylated IgM subunits were still able to inhibit specifically the homologous precipitation in the U-pAla-pLys -IgG antibody system. The average association constants for uridine of both IgG and IgM a.ntibodies, measured by the equilibrium dialysis technique, were low and of a similar order of magnitude.The complement fixing ability of purified IgM and IgG anti-uridine antibodies upon reaction with the homologous antigen displayed significant differences : low complement fixation activity was found in the IgM fraction either a t 4" or a t 37", while a high activity was found in the IgG fraction a t both temperatures. The IgM fraction lost all of its ability to fix complement when the macroglobulin was reduced and carboxymethylated. PCA reaction, using a highly immunospecifically purified I9 S anti-uridine antibody, was found to be negative when assayed up to an amount of 100 pg.

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