Epizoovotiologic studies were carried out at two industrial pigbreeding complexes for 40,000 and 33,000 animals with a record of leptospirosis. The serologic examination of pigs made use of the microagglutination reaction in the investigation of a total of 3,793 blood samples by groups corresponded to the technology of raising introduced at such complexes--sows, growing animals, replacement animals, and pigs for fattening, along with the study of 20,914 blood samples taken from pigs in various regions of the country. The total number of positively reacting pigs accounted for 12.9 per cent. Highest was the percent of positively reacting sows and growing pigs--19.1 and 12.9, while the replacement animals showed 8.31. No positively reacting animals were found among the pigs for fattening. Highest titers were noted among sows--up to 1:12,800 (infectious), and lowest among growing pigs--up to 1:100 (postnatal). State are the substantial economic losses caused by abortions and stillborn piglets--11 and 23 per cent, resp., for the two complexes. The basic Leptospira type for the two complexes and for the country as a whole proved L. pomona. The main source of infection were the carrier pigs that excreted Leptospira organisms, and the infection at the industrial complexes came from the hybrid sows and boars imported from abroad for grading purposes. A biologically justified and economically expedient programme of immunoprophylaxis in the control of Leptospirosis was worked out, providing vaccination of replacement pigs and sows only, treatment taking place after impregnation.