To study the expression pattern of immunoglobulin-encoding genes in transgenic animals, we have introduced the genes for the light and heavy chain of a mouse monoclonal antibody (mAb) into the germ-line of mice (control), rabbits and pigs. The transgenes were detected in the mouse lines, two rabbit lines and pigs. Titers of 100-200-mu-g mAb/ml (rabbits) and up to 1000-mu-g mAb/ml (pig) were measured in the sera of the transgenic animals. Isoelectric focusing experiments with serum followed by immunofixation revealed that in the transgenic pig only a minority of the bands were identical to those of the purified mouse mAb. In transgenic rabbits we found no coincidence of bands at all. The results can be explained by assuming tissue- and cell-type-specific glycosylation, modification and possible heterologous chain associations. Expression of Ab in the serum of animals could help to protect against diseases (e.g., influenza in pigs).