Standard CD44 (CD44s) and variant isoforms (CD44v) are expressed on different malignant cells and tissues. Their upregulation has been implicated, in the progression and metastasis of malignomas. In this work we addressed the question of whether these molecules are also expressed on xenografted human breast carcinomas and if certain expression patterns are correlated with biological parameters like tumour size, hormone receptor status, histology, growth rate, chemoresistance and microenvironment. Additionally, we were interested in the shedding of soluble CD44 (sCD44) into the blood circulation of tumour bearing nude mice. The human breast carcinomas MCF-7, MCF-7/ADR, 4296 and MDA-MB435, 4134 and 4151 were transplanted subcutaneously (sc.) or into the mammary fat pad (mfp.) of nude mice. The expression of the CD44s, -v6, and -v9 isoforms was determined at different time points on tissue samples by immunohistochemistry or RT-PCR employing human-specific antibodies or primers, respectively. The serum concentration of CD44s and -v6 was measured by human specific ELISAs. All tumours expressed CD44s. The lowest level was observed in the MCF-7 cancer. The CD44v6 and -v9 sequences and epitopes were distinctly expressed in MCF-7/ADR. MDA-MB435, 4134, 4151 and 4296, while MCF-7 lacked these isoforms. The highest serum concentration of the v6 isoform was detected in mice bearing the tumour 4296 with a high tendency for lymphogenic metastasis. The serum levels of sCD44 were in 5/6 xenografts linearly correlated with the tumour size. Interestingly, there was a remarkable difference between the two sublines MCF-7 and MCF-7/ADR: Both the tissue and serum levels of CD44 isoforms indicated that the development of multidrug resistance is accompanied by an alteration in the expression of membrane proteins discussed to be involved in metastasis. There was no relation of tissue expression with the transplantation site and the hormone receptor status of the tumour lines. CD44s and its variant isoforms are expressed in human xenotransplanted breast cancers in very different levels and patterns. The highest expression in the tumour 4296 is related to lymphogenic metastasis while the absence of isoforms in the model MCF-7 is related to non-metastatic behaviour. CD44 is shed into the serum and can be used for monitoring of tumour growth.