A method is described for production of an equine anti-bovine leukocyte serum (EABLS). Leukocytes were harvested from the milk of cow''s udders which were irritated with [Escherichia coli] endotoxin. The washed leukocytes (as antigens) were administered to 2 horses in a series of s.c. and i.v. injections. There was a variably progressive increase in total serum proteins and a decrease in albumin/globulin ratios, but the most pronounced change was an increase in .beta.2-blobulins. Accompanying these changes was an increase in the number of precipitin lines as shown by Ouchterlony analysis. Four old cows and 2 calves were given [i.v.] a 50- and 20-ml dose, respectively, of the EABLS. Two of the cows and 1 of the calves were given the EABLS which was heated to 56.degree. C for 30 min. Both calves and the 2 cows given heated EABLS and 1 cow given unheated EABLS developed marked neutropenia within the 1st hour. The 4th cow given unheated EABLS developed only slight neutropenia. It was concluded that there may be variation in the individual animal''s susceptibility to the neutropenia-inducing activity of EABLS.