After observing the nonspecific expression of the alpha S1 casein gene in peripheral blood mononuclear cells of goats, we continued to trace illegitimate gene-expression patterns in mammals. Six mammalian species (Bos taurus, Capra hircus, Ovis aries, Equus caballus, Sus scrofa, and Bison bonasus) were analyzed for the presence of kappa casein in peripheral blood mononuclear cells and Ficoll-isolated lymphocytes. To detect potential relationships between the expression pattern and reproductive status of the individual, the animals within species represented different sex, age, and physiological status (immature and reproducing males; immature, pregnant, lactating, and sterile females). Species-specific primers were designed for the two-step post reverse transcriptional-polymerase chain reaction. The products were sequenced and a Blast search performed. Transcripts of kappa casein were successfully detected in the peripheral blood mononuclear cells and lymphocytes of three of the species (goat, cow, and European bison). The presence of the transcripts was unrelated to sex, physiological status, or age of the animals.