1. Pairs of blood samples were taken from individual Periplaneta americana, the first sample being used for a live-blood haemocyte count and the second (collected 10 sec. later) for potassium and sodium determinations.
2. Analysis of these matched data on cell count and whole-blood ion concentration reveals a positive correlation between the haemocyte density and the potassium concentration, the mean regression coefficient being +0·83 mM K+/l. of blood for 10,000 cells/µl.
3. Similar analysis of the sodium data gives a negative correlation, with a mean regression coefficient of -1·69 mM N+/l. for 10,000 cells/µl.
4. Haematocrit estimations on heat-fixed blood, whose haemocyte density was simultaneously determined, indicate a mean haemocyte volume of 720 µ3.
5. If the correlations relate entirely to the contents of the haemocytes, comparison with the haemocyte volume would indicate that the cell sap contains 115 mM/l. of potassium and probably less than about 60 mM/l. of sodium; these levels are very similar to those reported for other tissues of the cockroach.
6. Since cockroach blood contains relatively little potassium these figures suggest that a significant proportion of the whole-blood potassium may be contained within the haemocytes and so not be immediately available physiologically.