The response of the ionospheric plasma to multiple pumps is seen in the enhancements of the incoherent scatter plasma line. Two strong HF pumps are transmitted, along with weaker sidebands. The two main pumps have a frequency separation ranging from 0 to 7 kHz. At zero frequency separation the standard decay and purely growing mode instabilities are observed. When two pumps are present, the normal decay instability usually disappears, and is replaced by strong oscillations at the pump frequencies plus features half way between pairs of pumps. These ''arithmetic mean'' peaks appear in most of the cases observed. The weak transmitted sidebands seem to influence greatly the observed spectra; the theory is treated in a companion paper.