Polymorphism of the casein gene in goats determines milk processing quality and cheese flavour. The main 7 alleles belong to 4 groups: strong alleles A, B, C (which code for 3.6 g/l), medium allele E (1.6 g/1), weak alleles F and D (0.6 g/1) and zero allele, connected with lack of alphaS1 casein in milk. Milk cells (mononuclear milk cells plus epithelium cells) and peripheral blood mononuclear cells were mRNA sources for reverse transcription. Three specific primers were used for polymerase chain reaction, which enabled to differentiate between alleles from four expression groups. The length of PCR products varied since allele F has a 111- nucleotide (nt) deletion of exons 9-11, allele D has a deletion of 36 nt (exon 9), and the medium allele E is associated with a 457 nt insertion in the 19th non-coding exon. Sequencing of amplified fragments, performed on PCR products isolated from milk, confirmed the correctness of the RT-PCR - based alphaS1 casein genotyping method.