To evaluate two in vitro assays for their ability to detect known developmental toxicants and nontoxicants, a series of 44 coded compounds were assayed by two independent laboratories using standardized protocols. The two test systems were (1) the human embryonic palatal mesenchymal cell growth inhibition assay and (2) the mouse ovarian tumor cell attachment inhibition assay. After all compounds were tested, they were decoded and ranked according to the minimum IC50 value (the millimolar concentration of compound which inhibits growth or attachment by 50% compared to the solvent control) from either test. The in vitro test result concordance with established in vivo animal and human test results was examined over a wide range of concentration levels (above which the in vitro results were called positive and below which they were considered negative). A positive response from either test was defined as a positive in vitro response. Concordance was defined as the number of correct responses divided by the number of chemicals tested. At the 1 mM level, the concordance of data from the combined in vitro assays with the in vivo data was 66% in one laboratory and 58% in the other. The maximum agreement between the combined in vitro and in vivo data was reached at the 20 mM level, where there was a 73 and 74% concordance of results in the two laboratories. At that level there was a 16 and 10% incidence of false negative results, and a 54 and 77% incidence of false positive results. A portion of these false negative compounds may require metabolic activation. The use of either assay alone was not as accurate as using a positive result from either test. Agreement of the in vitro data at the 10 mM level with available human data was 71 and 75% for each laboratory. The data indicate that the two assays are complimentary and as such the combination of these assays could be useful as a preliminary screen to establish priorities for in vivo developmental toxicity testing.