Early pregnancy factor (EPF) is a protein detected in the serum of pregnant females in most mammals, before implantation of the embryo; its activity is detected by the rosette inhibition test (RIT). After preincubation of lymphocytes in serum of non-pregnant pigs, the mean titre of RIT was 10.80 +/- 0.32. The upper limit of the 95% confidence interval was calculated to be 11.44. Therefore, all titres 12 were defined indicating the presence of EPF activity. The incubation of lymphocytes in serum of pregnancy caused a significant increase in the mean rosette inhibition titre, raising it to 19.88 +/- 1.13 (p < 0.001). False negative results were obtained for 10% of pregnant pigs. EPF positive activity was detected in only one non-pregnant pig (RIT = 20). This assay is not quantitative nor suitable for pregnancy diagnosis taking into account RIT values dispersion.