E. coli were grown for several generations in a medium with 15N. The DNA of the resuling cells had a higher density (was heavier). After that, E. coli cells with only 15N in their DNA were put back into a 14N medium and were allowed to divide only once. DNA was then extracted from a cell and was compared to DNA from 14N DNA and 15N DNA. It was found to have exactly an intermediate density. This supported the idea of semiconservative replication.
The DNA was intermediate in density because it had an all 15N DNA strand and an all 14N DNA strand. The all 15N strand was one of the original strands in the original cell. The all 14N strand was a newly synthesized strand.
Trivia : Meselson's external advisor for his thesis was Richard Feynman. Feynman quotes this experiment as one of his reason's for staying at Caltech. Feynman had just got an offer from the east coast and was seriously debating going there when Meselson approached him with the results of this experiment. Feynman thought there is no way this can happen anywhere else, so decided to stay!!