The present research had two objectives. The first was the isolation and the molecular characterization of strains of small ruminant Lentiviruses from mixed flocks of sheep and goats, as well as the comparison of a part of the gag gene which encodes the structural protein p25 both among themselves and with the prototype strains. The second objective was the random recording and the comparison of the percentage of seropositive sheep and goats, in mixed flocks with a history of progressive pneumonia, as well as the investigation of the frequency of seropositive animals which were clinical healthy. For the first objective two mixed flocks of sheep and goats (from the region of Thessaloniki) with a history of progressive pneumonia in sheep were used. Three sheep with clinical symptoms of progressive pneumonia and three goats were selected from these flocks. These animals were both, serologically (Agar Gel Immunodifussion Test) and LTR-PCR (Long Terminal Repeat-Polymerase Chain Reaction in blood leukocytes) positive. For the isolation of the virus from each animal, monocytes from the peripheral blood were isolated and cocultivated with cell-cultures from choroid plexus of a healthy lamb. The cells were collected upon the appearance of the cytopathic effect. The total DNA was extracted for the detection of the proviral DNA. A PCR method was applied with a pair of primers for the gag gene of DNA provirus. The virus was isolated from all three sheep and only one goat. During isolation it was observed that the viral strains had different biological properties: The sheep strains had cause severe cpe, while nothing was observed in the goat strain. The nucleotide and the deduced amino acid sequences of the amplified proviral DNA were compared among the Greek strains and with the prototype strains K1514, SA-OL, EV1 and CAEV-CO in order to verify the degree of resemblance. It was observed that the sheep strains were similar compared among them (96-98% in DNA and 99-98% in aminoacids) and less similar compared with the goat strain (90-92% in DNA and 95-96% in aminoacids). It was also observed that there was greater degree of resemblance with the prototype sheep strains (K1514, EV1, SA-OL) than with the prototype goat strain CAEV-CO. For the second objective a serological examination (AGIDT) was done in 286 sheep and goats from 7 mixed flocks of N. Greece with a history of progressive pneumonia in their sheep population. Percentage comparison were made with respect to seropositivity and the presence or absence of clinical symptoms. It was observed that the seroprevalence in sheep was 49% and the seroprevalence in goat was 60%. It was also observed that 47% of seropositive sheep were appeared as clinical healthy. The seropositive sheep appeared symptoms in 22% while none seropositive goat appeared any symptoms. In addition, LTR-PCR was done directly in the peripheral blood leucocytes of 76 out of the 286 sheep and goats. The results of the AGIDT and the LTR-PCR methods were then compared so as to determine sensitivity of the two methods for the detection of infected animals. It was observed that, in agreement with other references the serological method was more sensitive than the LTR-PCR method (the sensitivity was 88% for AGIDT and 79% for LTR-PCR).