Biological control agents (BCAs), and among them some species of fungal endophytes, are potential substitutes for chemical pesticides in the control of plant diseases due to their lower toxicity to human beings and their surrounding environment. One mode of action of fungal BCAs is through their bioactive, extracellular products, which can inhibit the growth of pathogens. In this study, the effect of fungal filtrates from four endophyte isolates (Trichoderma viride, Aureobasidium pullulans, Aureobasidium sp. and the unknown endophyte 20.1) on the advance of the pathogen Gremmeniella abietina on two-year Pinus halepensis seedlings was evaluated. Both preventive and therapeutic treatments of the filtrates were studied by applying the filtrates either before or after the pathogen inoculation, respectively. In order to verify the results, the entire experiment was repeated. Six months after the inoculations, symptoms of the disease started to appear, and the seedlings were processed. Since G. abietina is a necrotrophic fungus, the length of the necrosis produced by the pathogen was used as response variable in our experiment. As a first step to exploring the chemical composition of the fungal filtrates, a simple HPLC screening of UV-absorbing components was conducted. The results of the study showed that all fungal filtrates were able to reduce the advance of G. abietina when compared to the control seedlings, regardless of the time of inoculation and the treatment. Low-molecular weight phenolic compounds could be detected in some but not all filtrates, warranting further studies on the possible role of these compounds in fungal filtrates.