A combination of selective enrichment by using immunomagnetic separation of F4 (K88)-positive Escherichia coli and a nested colorimetric polymerase chain reaction (PCR) was used on crude clinical and spiked samples for determination of genes encoding heat-stable enterotoxins (STs) Ia (ST Ia) and Ib (ST Ib). The combination increased the sensitivity of the nested PCR compared with that of application onto crude samples. Dead cells were also enriched by use of this technology, giving results that are not available by traditional cultivation as enrichment before PCR. The second step in the PCR was modified to be able to differentiate between ST Ia and ST Ib genes. The colorimetric PCR was performed in a microtiter format, making it useful for automation in clinical laboratories and for the screening of large numbers of samples.