The study included a sheep flock comprising five genetic groups. The ELISA was applied to perform constant monitoring (every six months) for the infection of ewes with small ruminant lentivirus (SRLV). The research results demonstrated a negative effect of SRLVs infection on lamb rearing that, depending on the genetic group, proved to be lower 1.3%-1.4% compared to the seronegative mothers. At relatively equal fertility (94%–100%) and more differentiated prolificacy (179%–198%) in all the examined groups (except the Suffolk breed), a rearing index was higher in the seronegative animals 6.8%–24.1% compared to the seropositive mothers. The Suffolk breed proved to be the genetic group most susceptible to SRLV infection. A prolificacy of infected ewes was 10% lower, a lamb rearing rate was 13% lower , and a general reproductive performance was 18% lower in comparison to healthy ewes.