An assay for chemosensory responses by the ciliate Tetrahymena thermophila is described that uses glass capillaries with a rectangular cross-section (inner dimensions, 20 x 2 x 0.2 mm). These have optical and geometrical properties permitting convenient observation of cell behavior within the capillaries.

Washed cells, starved for 12 h, accumulated preferentially in capillaries containing L-methionine, L-leucine, L-cysteine, L-histidine, L-histamine, cimetidine, agmatine, and berenil at concentrations of 10^-3 M or less. They avoided capillaries containing tripelennamine, diphenhydramine, and pentamidine at these concentrations. It is argued that the actual response thresholds are much lower than the concentrations put into the capillaries, since cells respond to the gradient of the diffusing chemical.

L-Isoleucine, itself inert, blocked the response to L-leucine but not to L-methionine, L-cysteine, or L-histidine. L-Ethionine and 1-homocysteine caused accumulation but not L-cysteine or DL-cystathionine. L-Cystine did not block the response to L-cysteine.

Cells accelerated when entering a capillary where accumulation occurred. On reaching the interior they swam more slowly and uniformly, and with fewer turns or stops than in control capillaries lacking the chemical signal, or when outside of the capillaries. Cells were inhibited from leaving both control and test capillaries, possibly because of accumulated wastes or secretions in the surrounding medium.