Melanoma cell invasion in vitro was tested by means of confrontation cultures of melanoma multicellular spheroids with rounded fragments of embryonic chick heart tissue. Quantitative determination of invasion was performed using a computerized image analysis program, facilitating the evaluation of the efficacy of potentially anti-invasive compounds. Retinoic acid (RA; 1 microM) [corrected] considerably impaired K1735-M2 melanoma cell invasion, as demonstrated by various measuring parameters. Parameter TUMAREA, expressing the amount of tumor tissue, indicates a growth inhibitory effect and the invasion parameter STRCSTR shows that after treatment with RA the stromal component was better preserved than in untreated controls. Besides the inhibitory effect of RA on melanoma cell invasion in confrontation cultures, RA increased the dynamics of adhesion of melanoma cells to the extracellular matrix components type I collagen and laminin, and slightly impaired melanoma cell directional migration. Fluorescence microscopy using rhodamine-labeled phalloidin showed that RA also modulated the organization of the actin cytoskeleton by inducing the formation of actin-containing stress fibers. Our data show that 1 microM RA exhibited a pronounced anti-invasive effect on highly metastatic melanoma cells in vitro. Impairment of host tissue degradation, altered adhesion abilities, changes in the actin cytoskeleton, as well as the antiproliferative effect may all account for inhibition of melanoma cell invasion.