Primary cultures of surgery samples from 9 human breast carcinoma cases were studied by double staining with fluorochrome labelled phalloidin for F-actin and immunofluorescence by tubulin, alpha-actinin or vinculin. Primary cultures of normal human mammary gland and benign breast tumors were used for comparison. Results showed that microfilament cables (stress fibers) and microtubules were reduced dramatically both in number and organization in most carcinoma cells. F-actin aggregates appeared. They are dot-like and 0.1-0.4 microns in size. These F-actin aggregates were co-stained with alpha-actinin and vinculin, located at (near) the innerface of cell membrane at the cell bottom (cell-substrate contact). In individual cells, the aggregates varied in number from 10 to more than 100 per cell, tended to group into pad-like patches. The percentage of F-actin aggregate positive cells in cell population of each case varied from < 5% to 90%. There was a positive correlation between the frequency of F-actin aggregates and the level of clinic and pathologically diagnosed invasion and subaxillary lymph node metastasis. These results suggest that aberrations of cytoskeletal organization of microfilaments and microtubules may be a common phenotype in human breast carcinoma cells. The varied frequency of F-actin aggregates may reflect the different potential capacity of malignant cells to invade and metastasize, thus provide a diagnostic clue as well as suggestions for further genetic diagnostic investigation studies.