It has been previously reported that 8701-BC cells, derived from a primary carcinoma of the breast, constitutively express parathyroid hormone (PTH)-related peptide (PTHrP) and PTH/PTHrP receptor (PTH/PTHrP-R) genes, that N-terminal, mid-regional and C-terminal immunoreactive PTHrP can be found in cell conditioned medium and, furthermore, that exogenously added PTHrP (1-34), (67-86) and, to a minor extent, (107-139) are anti-mitogenic but promote Matrigel invasion by this cell line. It has also been reported that PTHrP gene expression is selectively switched on in those 8701-BC clonal lines endowed with a higher proliferation rate and invasive ability in vitro. Here we have first examined the presence of PTH/PTHrP-R transcript in the different 8701-BC clones by PCR and Southern blot analysis. Second, we have studied the growth and invasive response in vitro to PTHrP fragments by some of these clones, i.e. BC-3A, BC-61 and BC-66, selected on the basis of their lower (BC-3A) or higher (BC-1 and BC-66) Matrigel invasion ability and their expression of PTHrP (positive for BC-61 and BC-66) and PTH/PTHrP-R (positive for BC-61). Our data show the existence of clonal heterogeneity for PTH/PTHrP-R mRNA and for the proliferative and invasive responses elicited by treatment with diverse PTHrP fragments. In particular: (i) the sensitivity to PTHrP (1-34) is restricted due to the uneven expression of PTH/PTHrP-R; (ii) BC-3A cells (the less 'aggressive' clone) are resistant to the anti-mitogenic effect of the PTHrP domains and, most noticeably, exhibit a growth-potentiating response to PTHrP (67-86) opposite to that found for both the parental 8701-BC cells and the two other clones; (iii) all PTHrP fragments tested induced the expression of a growth-restraining and invasion-promoting phenotype by BC-61 cells (one of the more 'aggressive' clones). Present data in vitro support the hypothesis that in vivo PTHrP may be a key element in local control of the invasive process during breast carcinoma development and that its role may be, in turn, dependent upon the biological characteristics and the level of malignancy of the target cells within the multiclonal population of a primary tumour.